I was able to get to the bottom of this.
Here’s the line that was causing trouble: discourse/groups_controller.rb at e2e13a70f63605b92b4e521cc9f818061c1ccfb6 · discourse/discourse · GitHub
@Benjamin_D got it right. It’s a TL issue.
There’s a site setting min_trust_level_to_allow_invite, this was set to TL2 while the group owner had TL1.
Should a group owner be allowed to invite users to the group without meeting the trust level req? Seems confusing.