add_participant_to_thread
Adding user to existing conversation
REQUIREMENTS
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<?php
// Returns array of user ids (int), returns empty array if no participants or thread not exists
$user_id = 3;
$thread_id = 6610;
// Returns true if user was added or false if user was not added
$user_added = Better_Messages()->functions->add_participant_to_thread( $thread_id, $user_id );