Joining
If you are the admin of a server that supports activitypub relays, you can add this relay to your server.
Mastodon
Mastodon admins can add this relay by adding
http://relay.forfun.su/inboxin their relay settings.
Pleroma
Pleroma admins can add this relay by adding
http://relay.forfun.su/actorto their relay settings.
Others
Consult the documentation for your server. It's likely that it follows either Mastodon or Pleroma's relay formatting.