[support] Collaborative writing in Groups?

[pyg]

Hi,
I'm looking toward a solution to allow users of specifics groups to
edit posts made by other members of the same group.

Use case:
I have a Group "Foo".
Users "john" & "Alice" belong to "Foo", but user "Bob" don't
John add a new page to Foo (not public)
Bob don't see the new page, but Alice do : great :)

But :
How could Alice edit John posts (not using comments, just use the "edit" tab) ?

Is it possible using OG & contributed modules ?

I tried some configurations with OG, Content type, & workflow module,
it almost works except that Bob still have access to
/node/myprivatecollaborativepost/edit (but not to
/node/myprivatecollaborativepost )

Thanks for any help

Pierre-Yves