Basically everybody's name goes into a hat (literally a red top hat - I use the red one because it feels more christmassy) and I mostly randomly assign people a comic to do a guest page or illustration for. What the recipient decides to do with their gift is entirely up to them. Stick it up as a normal page, put it in the commentary for a page, guest art section, wherever they feel it should go.
You can pick the maximum age rating you wanting to draw something for, which is where the mostly random part comes in. All ages comics go in the hat first, then PG, then teen, etc.
As well as signing up as a full participant you can sign up as a safety net elf. Literally anyone can sign up for that. That's where you volunteer to take over an assignment in case someone drops out. They're not usually needed and I think the record in all the time it's been going has been two safety net assignments given out in a year.
I'm a safety net elf, because I can't really randomly assign my own comic and then be surprised by who emails me a drawing.