Currently, studio uses the following code or something similar in a number of places:
return userService.getOpenIdDomain(userService.findCrowdUserByName(getEditName())) != null

A lot of code will be cleaner if the API allowed this to be performed in a single call.
Add a method with the following signature to the com.atlassian.agmp.openid.users.mgmt.api.UserService interface:
boolean isOpenIdUser(String username) throws com.atlassian.agmp.openid.users.mgmt.api.UserNotFoundException