In Cloud SaaS mode, we allow free text addition of cloud users to collections. However, this only works for User IDs. This doesn't make sense in a Cloud SaaS environment as users never know the IDs of users to search with. We should therefore limit the selection of collection users to those that exist in the cache and prevent the addition of users using free text. The backend will no longer even attempt to resolve these users by fetching going forward. Possibly some design work will also be needed for this as I don't think there is precedent elsewhere in Optimize.