Create a new "pending" group of organization members

[stephanegigandet]

What

• When users create an account on OFF, they can indicate which organization they want to join.
• If the organization already exists, we currently don't add them to the organization, we just send an email to OFF producers admins.
• Instead we could create a "pending" group of users in each organization, and add users to it. That way we could then display the list of pending users to the org admins, and allow them to add or remove pending users to the organization.

Part of

• 🏭 Producers platform (Tracker issue) #5521