Adds a user as a group member. The user must not already be a member of the group.
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.
application/json The ID of the group
"123e4567-e89b-12d3-a456-426614174000"
The user identifier, which must be exactly one of the following: Slack user ID, email, or Tightknit profile ID
User is already a member of the group