I'm guessing you mean an
Accommodation Certificate ("AC"), rather than a Guest Certificate. A Guest Certificate is something you purchase from II for $39 that allows someone else to use a week that you exchanged through II.
See the above link for an explanation of how an AC works. Essentially, the AC allows you to purchase a week from part of II's inventory of available weeks for an additional week of vacation. For the most part, the weeks you can purchase are off-season weeks.
The only weeks that you can deposit with II to request an exchange are weeks that you own. Thus, if you reserve your own Manor week with Marriott, you can deposit that week with II and will have until two years after the Manor week dates to make and occupy an exchange.