Presence

A presence channel (presence-…) lists who is on it, to everyone on it: who is reading a post, who is in a support queue, who is typing.

Who is a member

Your callback decides, as for a private channel (channels), and also says what the others see:

wordplus_realtime()->channel( 'presence-board-', function ( $user_id, $channel ) {
	if ( $user_id <= 0 ) {
		return false;
	}

	$user = get_userdata( $user_id );

	return array( 'info' => array( 'name' => $user->display_name, 'avatar' => get_avatar_url( $user_id ) ) );
} );

• A member is a signed-in user, under their user id, with their display name as info unless your answer gives other info.
• A visitor needs an id of yours: answer array( 'id' => 'guest-81f2', 'info' => … ). A visitor without one is refused a presence channel (forbidden).
• info is any JSON up to 1 KB. Everyone on the channel sees it, so put nothing in it the channel's members shouldn't see. It travels sealed with the channel's key, so the realtime servers don't (encryption).
• For a user's name and avatar on any page, whatever the channel, read the site's directory with rt.users( ids ) (encryption).

In the browser

const board = rt.subscribe( 'presence-board-1' );

board.on( 'realtime:subscribed', ( members ) => drawMembers( members ) );   // [ { id, info } ]
board.on( 'realtime:member_added', ( member ) => addMember( member ) );       // { id, info }
board.on( 'realtime:member_removed', ( member ) => removeMember( member.id ) );

board.members();      // everyone on it now: [ { id, info } ]
board.memberCount;    // how many, counting those not listed

• The first 100 members come with the subscription, and memberCount counts them all. Members who join later are added as they come.
• A user is one member however many tabs, devices or plugins they're on it from. realtime:member_added comes with their first connection, realtime:member_removed with their last.
• Moves don't show anyone leaving. When the connection moves to another server, the new connection joins before the old one leaves.
• A server that dies has its members taken off by the cluster within seconds.

From your server

channel_info() reads a channel as the server holds it:

$info = wordplus_realtime()->channel_info( 'presence-board-1' );
// array( 'ok' => true, 'subscriptions' => 3, 'members' => array( 'count' => 2, 'list' => array( array( 'id' => '7', 'info' => array( 'name' => 'Ann' ) ), … ) ) )

Limits

1,000 members a presence channel; past it, a subscription is refused with full.