And if you struck up Diplomatic ties with this new faction, you could provide a sort of Prestige interaction, where you would send groups of Peasant Soldiers to their city, which could then be disbanded, and as such become new citizens of that city/faction.
That would provide your Ally more people for them to use to either grow their city, assuming that increasing Prestige level would be more difficult for them to do than real factions, or use in army creation tio be used with their allies, (you) in joint ventures down the road.
One would have to be rather careful with providing to many citizens to quickly though of course. One would hate to lose their allegance, only to have to fight off what used to be your own people.