post
https://api.qfp.quantnetwork.net/api/tenants//groups//members
Adds one member by user id. Idempotent: adding a member who is
already in the group answers 200 and changes nothing.
The user must exist in the caller's tenant (another tenant's user
reads as 404) and be ACTIVE or INVITED; DEACTIVATED and
REMOVED users are rejected with 400. A deactivated group cannot
gain members. Requires group:manage, and the path tenant must equal
the caller's tenant_id claim.

