chmod g+s /home/myname/data
What it does is force any files created by any other user in that group to have the same group ownership as the dir.
Note that it would be unusual to use this in your personal home dir.
Its usually used as part of creating an acct for a project group, so they can safely/easily share files.
You'd have owner = myproj, group=myproj and add the group s flag to the perms as above.
Note that owners and groups are in separate name spaces, so they can have the same 'name'.
Usually only the proj/team lead will have the proj acct passwd; no need for anyone else to know it.