Add a new member to the account. There are three ways:

• Email invitation — pass email only. An invitation is sent; the recipient accepts it from the dashboard. The member appears in pending status until accepted. Equivalent to calling POST /api/v1/invitations directly.
• Direct add — pass target_user_id for a user who already has an account on Raff (e.g. a project-only user being promoted to account member).
• API key — pass api_key_id to make an existing API key a member with the given role. Mutually exclusive with email and target_user_id.

{
  "data": {
    "role_name": "Operator"
  }
}