Hello,
On our forum (hosted by Discourse), we’re also seeing the same issue. Our site has “invites” (for users to allow to send invites to non-users via email) disabled, and personal messages now only show a “Remove…” button (for non-admin users). Briefly looking at the code linked here
it would be my guess that there is no dedicated code (function/branch) to test whether a user has the permission to invite others to an existing personal message. Maybe this “test” was originally handled incidentally one way, and after a recent update the behavior changed, but unintentionally (as a a side effect)? It would be great if this could be promulgated into its own proper settings on the admin dashboard (“Allow thread starters to add users to existing personal message thread?” and “Allow any participant to add users to existing personal message thread?”)…