Currently, if the option is set, the minimum setting for “Required trophies” to join an alliance is 200. That seems high and bit restrictive since there are lots of new members who want to join them.
Why not include in the options:
Additionally (this may be implemented already; I never checked) if someone tries to join an alliance but does not have the required number of trophies, the system should respond telling the person to fight in raids to increase trophy count.
The idea behind such a low number is to keep people who have never won a raid (or are terrible at them) from joining an alliance while still leaving joining easily open for everyone else.