each member that completes thru zone 30 of the Symbiote incursions event adds a point, so if 15 members do it, including yourself, you will get all the rewards.
I’m wondering if I join a new ally would I have to reach zone 30 again, and would I get all the rewards
Odds are you will not be able to score another point in a new alliance since it states that it can only be scored once. So unfortunately you will miss out on these bonus rewards without having already been in an active alliance.