curl --request POST \
  --url https://api.example.com/api/v1/contacts/groups/{group_id}/members \
  --header 'Content-Type: application/json' \
  --data '
{
  "contact_ids": [
    "3c90c3cc-0d44-4b50-8888-8dd25736052a"
  ]
}
'

{
  "success": true,
  "added": 123,
  "member_count": 123
}