@Amiee_Kane - does your organization use SSO? If so, your best bet is to manage access in that IDP. People will still be able to invite users in Asana, but those people won’t be able finish creating and access their account if they aren’t provisioned in the IDP group.
As you mention, long-term, you’ll want to talk to your Asana rep about maybe changing to a different model (e.g., free organization with paid divisions) where you’ll have more control in-platform on who can invite people onto specific paid teams. I assume you are currently on a paid organization, where I don’t believe you can limit invitation of users within your domain. For more insight on divisions, check out this post: