Each co-op has their own policies but generally there are databases' set up and you add yourself to them saying how much you want of whatever the database is. When the database closes (sometimes there are amt. limits (like you have to order 40 yd of something as a whole) or time limits (like they close on such-and-such date)) you pay for your item and when it comes in to whomever is running the co-op, is cut, etc. then you pay for your co-op fees and shipping to you.
When you ask to join the co-op you usually get plenty of this information and of course you can always ask questions... also spending some time and just clicking the links and seeing where they take you is a good idea (like how the "file" link usually takes you to specific information about co-ops that are going on, have gone on, etc., how "photos" can take you to detailed pics of the stuff going on now (or that have gone on)).
~Julie