Going to Special:InviteSignup shows You do not have permission to invite users to sign up to this site, for the following reason: You are not allowed to execute the action you have requested.
Even though Special:ListGroupRights shows my user group for sure has the InviteSignup Any ideas?
Just tried logging out/in to make sure and still there's permission error
Are you using MW1.29? It might be a version issue.
I am not using InviteSignup much currently and I am not even sure it works properly with MediaWiki since the auth code rewrite. But you shouldn't see such errors. Maybe related to https://gerrit.wikimedia.org/r/c/303358/ ?
A wiki of mine exhibits a similar, and I believe related issue: Users in the "staff", "admin" and "bureaucrat" user-groups are able to send invitations, but upon following the invitation link, the invitee gets a message that they are not authorised to perform the action.
I experience the same with MW 1.31. I think this is either broken or broken for private wiki use with the ConfirmAccount extension used at the same time.