Wagtail Page Privacy 'Private, accessible to users in specific groups' as SAAS separation
Yes, that should work - I can't think of any issues that would prevent that approach from scaling to thousands of groups.
However, be aware that Wagtail doesn't currently provide complete isolation between sub-sites on the same Wagtail installation - users will still be able to see pages from other groups when they're choosing a page to link to.
Scenario: A User registers to my Wagtail site. This creates a Group, and a Page. This page wil be set Private 'Private, accessible to users in specific groups' The just created group will be set.
The user invites people to the group, and shares pages as childpages of the just created root-page, so only group members can acces it.
Would this scale? For like, hopefully, thousands of groups? Is this a way to separate content for a SAAS setup?