I add to the correct answer from grgarside that documentation from Apple can be found here http://help.apple.com/advancedserveradmin/mac/4.0/
I have to say the documentation is good but not excellent so here is a little "recipe" for the most common setup.
A common practice is to create
users, to create
groups, to assign
user can belong to none, one, many
groups of course) and finally to give
groups access to shared folders.
group is created from the Server application check the option Create a shared folder for this group. As you click Ok to confirm the group creation the folder is created as well into
Don't create yourself the folders inside
/Groups as this (may) mess things up (this happened to me).
You finally go to the Sharing pane and add the folders inside
/Groups to the list of shared folders.
Each shared folder will be automatically set to be read-writeable by its group.
Finally you turn file sharing on.