You're not messing anything up, but a few pointers to help you find your way round.
If you join a group, the work people want critiqued right now is in a list next to their names.
There's a forum part of the group at the top, but under that is a list of members of that group, and against each member's name it will either say the name of a piece of work, or "no work uploaded". The most recently uploaded works are at the top of the column, so those are the best to start critiquing. They will be the most current pieces.
If you click on the name of the work, you're taken to a page where the piece of work appears and all comments about it appear below the work.
My advice is that if you get in there and critique a few of the recently uploaded pieces to that group, then if you upload one of your own to the *same group* (that's vital), then you will very likely get some critiques in return.
I don't think people are against critiquing temporary members, but they are against temporary members uploading and expecting critique without having given any, and then disappearing, thus taking and not giving. So if you give first, you will then get people more than willing to give back to you.
I hope that helps.