Hey @gassim , thanks for your patience while we’ve been looking into this. I’m afraid the short version of this is that your use case is not really part of our intended design for invites, and it would not be easy to adapt invites to support this.
As designed, invites are meant to guide new users through the registration process, not to change existing users. We did some digging and believe that we made a security fix back in 2022 that disabled the functionality you’re describing here for existing users using invite links.
Invites are a tricky area for us because there are lots of security implications, so reintroducing this functionality is not a simple undertaking for us. We would need to significantly refactor invites to effectively support your use case of using invites for new and existing users.
I understand that supporting your use case is important and this information is likely disappointing to you! However, I suspect that what you want could be accomplished through some other means. I would start with checking out Discourse Automation — we have some docs on how to add users to group automatically which might fit your needs: Adding users to groups through custom field automation
If you need more help along these lines, please open a Support or Feature topic and we can pick things up from there.