Workshops on Neopoet are groups that meet for a certain period of time to focus on a certain aspect of poetry. Each workshop participant is asked to critique all the other poems submitted into a workshop. A workshop leader helps coordinate -- they set the agenda, give participants feedback on whether their submissions and critique are at they level expected of them, and after the workshop is over, give feedback to participants.
To join a workshop, first find one that is of interest to you. Once you have found the right workshop (and verified that it is open -- you can find this out in the description below), you can apply to join the workshop.