Endpoint to request to join a working group

[redfrexx]

We need an endpoint through which a user can request to join a working group. After the request is sent to the backend, the representative of the working group gets an email with a button the decline or accept the request. By clicking on either one the user is sent back to the app and the decision is sent to the backend where the working group of the user is either changed or not.

[redfrexx]

@codingfabi Which link should the user be sent to if they accept the request? And which one if they reject the request?