XMPP knows invite links, there are even servers which support invite pages which list suitable clients one could use for XMPP. Prosody for example has this module you could enable: mod_invites_page

And then there is also Snikket, which is a pre-configured XMPP server suite (running prosody under the hood) with some nice QoL improvements baked in which you can selfhost as well: Snikket Quick Start Selfhost