When you create your group, the 12th-14th (?) fields are
make sure Allow users to join the group freely (Requires publicly visible group) is selected and that Visibility is set to Everyone. Or if you want to only allow user to request to be added and not be able to add themselves, select Allow users to send membership requests to group owners
If you’ve already made the group, you’ll find these options under
/g/groupname
Manage → Membership → Access
Then once a user goes to the groups page they can see the group and add themselves.
