If you're asking about the Breeder group you click on the link your got in the email invite...that's the only way to join that one as it is a private group. Then click on the group to visit it (see tab at the top of the page for 'Groups' on our site. Otherwise you can join any of the others without an invite by going to the group and clicking on 'Join Group' :)
If you don't figure it out today...please describe exactly what you do (steps you take, things you click on, things you see) and what results you get with each step. That will help either me or another breeder member who sees your post better help you trouble shoot.