fun hasParticipant(participant: User, onCompletion: (Either<Boolean, SippoResourceError>) -> Unit): Unit
Checks if a participant belongs to this group.
participant - User to be checked.
onCompletion - Callback that will be invoked when the request is completed.
In case of success, it will receive true if the user belongs to the group
or false if they don't. In case of error, it will receive a SippoResourceError.