A couple of possible approaches that don’t use SSO:
-
Send users an invite to the forum through the Discourse API when the user signs up via Woo Memberships. It might be possible to use Zapier for this: Automate sending Discourse invite emails with Zapier
-
Create a Discourse user and/or add them to a Discourse group when the user signs up via Woo Memberships. This could be done with the Discourse API. It will be tricker to implement than just sending an invitation to the forum, but since you might want to also be able to remove the user from a group when their membership expires, it could be worth looking into this approach.