Can you clarify the above?
Do you mean that the users don’t already have Discourse accounts, or that you don’t want to use Discourse to send the invitations?
If it’s the former, then you can invite users from your profile and specify any group membership along with the invitation. You can also upload a CSV of email addresses and group memberships.
If it’s the latter and you’re using SSO, then group membership can also be provided in the SSO payload.