Is there some kind of "Scale up" configuration in ActiveMQ Artemis?

Can messages from a broker be sent to another broker when it joins the cluster?

As in the Scale Down config, when a broker shuts down, all the messages from that broker are transferred to another broker in that cluster; is there any configuration where, when a broker joins the cluster, the load among all the brokers gets divided?

No. There is no "scale up" procedure for new brokers joining the cluster whereby messages from the existing brokers are migrated/transferred to the new broker.

Implementing such a procedure is not nearly as straight-forward as it might first appear, and it's not clear that there's any real demand or a legitimate use-case for it.

You can potentially use a connection router in front of the cluster to help balance things out if necessary.

Generally speaking, you may find a new section in the documentation regarding cluster performance helpful.

With message redistribution Apache ActiveMQ Artemis can be configured to automatically redistribute messages from queues which have no consumers or consumers with filters that don't match messages. The messages are re-routed to other nodes in the cluster which do have matching consumers. To enable this functionality the redistribution-delay address setting must be >= 0 and message-load-balancing must be ON_DEMAND or OFF_WITH_REDISTRIBUTION, for further details see Message Redistribution.