I need to limit access to our idea portal to ~200 people in a ~900 person company- this is to protect IP that will be generated in the idea portal and keep employees busy with what they should be busy with.
Ultimately we will build several portals that will be used by different segments of the company- and we will need to limit each portal explicitly to it's intended audience- access to an email account on a particular domain along with SSO credentials is not enough.
You should be able to achieve this with private portals today. If you do not want self-registration then do not populate the "Employee/Partner emails" field in the portal configuration.
You can use CSV import to import the list of allowed users for each portal.
If you are using SSO then the SSO identity provider can limit access based on the domain of the portal and the group membership of the user (so no CSV import would be necessary).