You can “add” SMS users to iMessage groups, if the group consists entirely of phone numbers and you’re on a device that supports SMS.
The newly created SMS group doesn’t replace the thread associated with the original group, though, so it’s easy for the group to fork when this happens.
The newly created SMS group doesn’t replace the thread associated with the original group, though, so it’s easy for the group to fork when this happens.
Unlikely this will change with RCS.