To answer a few questions that I have seen:
Selection for Presidential Guard is usually done at boot camp. Senior Marines from the Presidential Guard program fly to each boot camp to screen marines for this duty. The initail screening process is long and eliminates most Marines. The screening process starts by bringing all Marines who are 03XX - Infantry to a breif. During the breif, the duty is explained and any Marine who does not wish to join is free to leave. The program is 100% volunteer. All Marines who wish to try out for the program have to meet certain criteria. The Marine can not be married or have any dependants. Criminal records must be relatively clean, but are considered on a case by case basis. The Marine must have a first class PFT score and not be over weight standards. Major debt also disqualifies you from the program. When your Marine makes it through the initial screening, they will proceed to the secondary screening. While at the second screening, each Marine is spoken to on a one to one basis to determine further eligibility. The final step is a formal interview with a board, consisting of officers and staff nco's of presidential guard. Questions during this interview are random and sometimes "off the wall" to determine how the Marine reacts to different situations. Bearing and trustworthyness are also evaluated during this time. If your Marine is selected for the program, the final step is to fill out the nessisary paperwork. One thing to note: marines serving in this duty must extend their contracts to 5 years active duty.
After SOI, your Marine will attend security forces school in chesapeake, VA. The school is 6 weeks long, but many marines get stuck there for up to 5 months. The school's class sizes are small and marines have to wait to pick up with a class. While waiting, some marines get RA while others do working partys.
After compleation of SF school, your Marine will be sent to Marine Barracks 8th and I in Washington, D.C. The time spent here all depends on how fast the Top Secret clearance processes. The clearances have become completely automated instead of paper, so clearances are going through much faster than they used to. The normal time is between 6 months and a year. Dont be worried if the clearance is taking a long time, the initial screening process disqualifies almost all people who will not clear.
While at 8th and I, your Marine will be performing guard duty at the base. They will learn more advanced techniques and mission specific rules during there time here. The work schedules are long and sometimes chaotic, but your Marine will have plenty of time to see all the wonderful things in the DC area. During parade season, guard works very long days, so dont be worried if you dont hear from your Marine as often as you would like to. It will get better as time goes on.
When your Marines clearance goes through, they will be transfered to one of a few locations around the DC area. The furthest away is Camp David, while the others are close to 8th and I. Most guard Marines go to Camp David, but the other locations are still very nice. Don't be offended when your Marine doesn't tell you nearly as much about their duties as before, almost everything is classified to protect security.
In total, presidential guard is a 3 year duty. The first year or so at 8th and I, and the remaining time at their final destination. After there 3 years, your Marine will PCS to an infantry unit and start their origional mos.
If for some reason your Marines clearance never goes through, they will be utilized at 8th and I for there three years. Some stay in guard, but most are sent to a different section, including; PMO - provost marshalls office. Here they will still work with guard, but not standing posts. They will deal with training of guard marines, overall security, and a variety of other missions. Another common place to be sent is Grounds. Grounds is a section of maintenance, made up of marines origionally from guard or marching platoons. Grounds is responsible for the upkeep of the barracks, ranging from general landscaping to setting up for ceremonial functions such as parades. While this is not your Marines primary MOS, grounds is a good place to be as they encourage their marines to attend school in one of the many local universitys or online.
Marines chosen for presidential guard are not allowed to be married or to have any dependants. If your Marine would like to be Married, they can submit for command approval once they have recieved their clearance and are transfered to one of the secondary locations. If your Marine does get married or adds a dependant while they are working at 8th and I, they will be removed from the presidential guard program and follow the same path as the marines who's clearance doesnt process above.
Presidential guard Marines will recieve extra uniforms when they arrive at 8th and I. They will get dress blues as well as dress whites. Marines at 8th and I are the only marines, under the rank of SSgt, who are allowed to wear ceremonial white trousers. It is considered an honor.
8th and I is a very small base, as mentioned before. The px at the barracks carries food, cleaning supplies, and a few uniform and recreational items. Henderson Hall has a full px where your Marine can purchase all uniform items and other usual px goods. Bolling air force base has a commesary which is like a grocery store where your marines can purchase food at.
All together, DC is a wonderful place to be stationed and is full of things to do. There are many museums, monuments, and historical places to see, as well as the opportunity to camp, fish, ride mountain bikes, visit amusement parks, and many other attractions close by. Vehicles are not as important as they are in other areas, because of the great public transportation system DC has. There is a subway which goes most places, as well as buses to many others.
8th and I also has some of the best medical care available in the military. Basic sick call is a couple blocks away at Washington Navy Yard, and more serious injurys can be handled at either Bethesda National Naval Medical Center or Walter Reed Army Medical Center. Both are the best hospitals the military has to offer, and most seriously wounded warriors will be sent there for treatment once returning to the states.
Lastly, Marines selected for presidential guard are considered some of the best the military has. To be accepted and recieve a Top Secret clearance is a great honor. Very few Marines have the opportunity to serve with presidential guard and only the top 0.1% of all Marines will have the chance. After successful completion of their dutys, Marines recieve a Presidential Support Badge. This decoration is a great honor, great enough that the badge representing this honor is serialized and issued to the Marine. These Badges can not be purchased anywhere, limiting counterfit use. The only way to recieve another PSB is to submit the proper paperwork and have a second serialized PSB personally delivered to you.
If there is anything else you would like to know, feel free to ask away! i hope this answers your basic questions!