If you aren't a member of any groups, click on "My Groups" to get started. This is the page you will see:

Click Search existing groups or Search for groups to find groups by searching words in the title or description of the group.
You can also search for the username of the group's creator (if you know it) by clicking on “Invite/Find your friends,” listed under Quick Links on the home page:

Simply click on the group's name in the search results:

Joining Groups
You can join most existing groups by simply clicking "join" from the group's main page.

If the group is not public you will need to apply for acceptance or get the password to enter the group from the creator or a group manager.

When you have been accepted into a group, it will show up on your "Groups" list on your dashboard or on your "My Groups” or “My Friends" page.