Actually, the support for basic external groups definitions and mapping is already in develop (1.5, in fact) via http://projects.theforeman.org/issues/5241 and https://github.com/theforeman/foreman/pull/1390. So the relation table is there and API support for manipulation of that mapping can go in independently of http://projects.theforeman.org/issues/813.

#813 adds a refresh action for external user groups, ensure this is added to the API.