When you register for a user, and enter their e-mail address, they will receive a link. If they click that link, they don’t need to confirm which project they are working on, since you have already confirmed their identity for them.
If they register using any other way, they will be asked for their PT7 registration code to confirm their identity (in the case of an existing user) or they could be creating a new registration, in which case they don’t need confirmation since they are not a member of any project yet.
If you are experiencing issues, please privately message me the name of the user so I can sort out what happened.