A tribe will typically have many followers, and only a handful of members.
To become a member, the Chief must either send you a tribe invite, or if you are always a follower of the tribe, they can promote you to a member from with in the tribe.
To promote a follower to a member:
- Navigate to the tribe.
- Click on the "Members" tab
- Click on the "Action" drop down next to the member you want to promote
- Select "Make Member"