Once on the main Networking Groups screen you will see the various categories of groups - as you see we have four categories:

 . 

You can join any of these groups by clicking the [Join This Group] link:

You can also search for the keywords in the search box on the Main Groups Listing.  A listing will show all the groups that contain that keyword.  Once you find a group, JOIN THE GROUP using the link next to the group name.

ALSO - When you view a Member's Profile, if that member belongs to a group that you would like to join - there will be a JOIN THE GROUP link right there next to the group name