When you checkin a file, you can make sure the new version is not the new member revision, for example if the changes are unfinished or untested.
What is the equivalent for adding/dropping a member? Is there a status for a file that says "I'm here, but not officially yet" or somesuch?
Yes there is the option of Update Member Revision (Shortcut: F7)
Basically, if you are checkpointing 1.2 member revision then it will become 1.3 and if you want to make again to 1.1 yes you can.
Please refer to below screenshot: