i.e. 'invite only registrations'
Currently you can only do a 3PID invite on a server if they have registration enabled (to convert the 3PID invite into a real user). 3PID invites could be specialcased to still work in face of this.

+1 Confirmed. As of riot v0.9.6 invite only registration doesn't work.
(added label from dup)
This is still not supported, which makes Riot a poor choice for private Matrix servers. It's been 3 years – any status update? I searched around but was not able to find shared secret registration support here on Github.
+1 for this holding me back from switching my friends to my private riot server
Most helpful comment
Currently you can only do a 3PID invite on a server if they have registration enabled (to convert the 3PID invite into a real user). 3PID invites could be specialcased to still work in face of this.