Group experiences’ Access Type can be set to Friends, but Groups do not have friends.
The only two options are either Public or Friends on the website.
In Studio via Game Settings you can set it to Private:
Expected behavior
I expect that in group experiences that you can set it to either Public, Friends/Affiliates (Affiliates for groups), and Private.
Page URL: https://create.roblox.com/dashboard/creations/experiences/${universeId}/access