Ensure invited portfolio members can see the org model

[abroddrick]

Issue description

When the org model waffle flags are turned on only for the org_model_user group in production
then as an project lead, I want users to be able to invite someone to their organization via the members portfolio table, and on that users next login they should see the org model view,
so that I don't have to manually add people to the org_model_user group when the member management flag is on

Acceptance criteria

• When someone is added to a portfolio they are also added the org_model_user group (see stable/staging or ag for the group name)
  • when the member management flag is on
  • note someone could be added to a portfolio with an invitation or user portfolio permissions, both should be covered here
• this applies to django admin and the registrar

Additional context

Right now on stable we are using this group to add people to the org model while still having the org model off for others, yet we didn't make sure that when a portfolio invitation is made that a person is added to this group.

removing someone from this group will be handled in #3318

Links to other issues

• relates to Remove a user from domains if they are removed from a portfolio #3318