LOE needed - private groups for 3rd party users

It would be helpful to have a rough, high-level predicted level of effort for future work so that we (Dan, Arti, Andrew) can prioritize the upcoming work based on both business priority as well as predicted LOE.

Private groups for 3rd party users: https://trello.com/c/JzVeDX6G/32-ability-to-create-private-groups-for-3rd-party-account-users-incl-activity-pages

Start by reviewing the linked Trello card for the proposed private groups for 3rd party account users, including activity pages. No repository files, tests, or entry points are named; done means producing a rough, high-level level-of-effort estimate for the described future work.

Written by the indexing model from the issue text.