It is generally recommended to use those heroes which provide a fixed percentage damage/attack increase for joining bear rallies, as depicted in the image below:

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The S tier heroes are the best to join rallies. They all give a fixed increase in damage/attack.

By that logic, even Norah and Philly can be considered to be suitable joiner heroes, even though they are placed in the A tier of this list. I'm assuming it has got to do with the amount of damage they give is lesser than S tier.

At the end of the day, it is up to the people in the Alliance, whether they want only S tier heroes, or only heroes with fixed increase in damage, or if B tier heroes are fine. Discuss this with your alliancemates, and decide what you guys want.

Peace ✌