fun add(participant: User, onCompletion: (SippoResourceError?) -> Unit): Unit
Adds a new participant to this group. Note: The user needs to be the group's owner.
participant
- User to be added.
onCompletion
- Callback that will be invoked when the request is completed.
It will receive a SippoResourceError if any error occurs. In case of success, error will be null.