Group on Group Conversion Rates

I have a question about group on group conversion rates. You are suggesting, once two larger groups are trusting each other f.e. Berlin and San Francisco they would convert their currencies 1:1.
But groups will emerge in different places and timespans, so each group will have a different economic activity in their respective groups.
One San Francisco Circle Unit wont have the same value as one Berlin Circle Unit. On the open trading market, each circle will have their own exchange rates to legacy fiat currencies or other cryptocurrencies.
1000 Berlin Circles = 10€
1000 San Francisco Circles = 1€

What is the incentive for the strong Berlin Group to join the San Francisco Group, as accepting a 1:1 exchange rate would dilute their own exchange rate?