We currently have a large set of users on mattermost and we would like to import these as users into our Discourse forum without requiring the users to accept an invite via email. I’ve tried to have a look for solutions but have not come up with anything yet.
Invites are probably the right way to go about this.
The point of accepting the invitation is that you’re getting their consent to add them to another system. You aren’t migrating them between discussion platforms, chat is a totally different thing.
All mport scripts provide examples of how that works with a forum migration. You could do it that way. You coyly also use the API. Cleaner might be to have your other system be the SSO server.