I managed to add users to a SP group using a temp table in between and addin a Flow that recats to the creation of a record in that temp table.
So, someone, via a Flow triggered in PowerApps, can request to be added to a SP security group. If approved, then the same Flow that manages the approval, add their email to another list. The second Flow is triggered wiht my (Admin) connections on Creation and the user is added via that second Flow.