Single button labeled sharing, or dropboxes.
Allow you to find anyplace that has sharing access for you.
Will make a list of all the folders that are set up to
be deliberately
shared with you, or groups that you are a member of.
This will be maintained by some form of database, which
will keep
track of these relationships.
It's OK for this database to lose consistency when the
filespace is editted manually.
Give each "dropbox" folder read access lookup
permissions for group
the "difficult" part is coming up with a simple ui
browse as "mfile", with a friend user piece of text.
"dropbox" directories will have permissions to an mfile
group (or user)