Greetings, fellow player.
I would join that tribe which has a low or significant number of members which is not near alliance capacity, as said my HP. If some tribe is like that, consider it as an MRA, or at least near to it. An ideal tribe has low number of members and excellent average point ratio. Also see whether the tribe is near to you or not. If it's not, then there's no point in joining it as you would receive support very late.
If no one is worth joining, simply don't join them. Wait for a few days and try to see the growth rate of that tribe. Also see the BP of the members of those tribes, as this would help in determining whether they are point whores or not...