Yep, that's correct. Implement a new membership provider, I think the easiest way is to use your new provider together with the EPiServerCommonIntegrationProvider that will copy the user and groups into the community tables. The alternate way would be for you to let your own provider copy the data to the community tables.
Please see this blog for information about the membership providers in community.
Thanks Tom. I'll look into this.
I want to authenticate against an external source and still have all community-user-functions with all user data and so on. What is the recomendation of how to solve this problem with EPiServer Community and its architecture? A new membership provider?