If you are invited to join a public group, you will receive an invitation to join with a link to the the group's context.
To accept an invitation:
1. Click the link included in the e-mail you received. In response, the system displays a screen with a portlet instance labeled Join This Group, which probably includes a statement that you have been pre-approved for membership or something similar.
2. To join the group, click Join Now.
Note also that when you are invited to a group, the system adds the group to the list in the My Groups view with a Join Now link. Clicking the link there is another method of joining.