Endpoint to let representative add users to the working group

[redfrexx]

Create endpoint which lets the representative add users to their working group if the user does not have a working group yet. Otherwise raise error.

[codingfabi]

For now, just create an input field where representatives can enter an email address. If this email address exists for a user in the database, the user is added to the working group. If not, we return an error to the representative.