after everyone who wants to do it has replyed saying they're going to, you decided who is Secret Santa to who. and when the cut-off date is for those who want to do it. you can participate too, only you're going to know who your Santa is...unless someone else chooses who is Santa to you. but anyway, the same people don't have to exchange between them, they can be mixed up. like, as an example, Simba is making Nala a gift, Nala is making Timon a gift, and Timon is making Simba a gift.
and anyone wanting to do it not able to get on Dec. 25th to post theirs should get it done earlier and send it to you (or whoever gets to pick who is Santa to who if you don't want to). sorry if I made it confusing, Moka, usually the admin likes to be over it

if you do it, when the cut-off date is reached you choose who will be Santa to who and PM each person with who they are going to be Santa for. just don't tell them who is Santa to them, or it wouldn't be Secret Santa.