What must be done to allow only users of the devel system group to access a share?

The `valid users` parameter, when prefixed with an `@` symbol (e.g., `@devel`), specifies that only authenticated users who are members of the corresponding Unix group 'devel' are permitted to access the Samba share.

`valid groups` is not a standard `smb.conf` parameter used for controlling which Unix groups can access a share; `valid users = @groupname` is the correct parameter and syntax.

`valid groups = devel` is incorrect syntax and not a recognized `smb.conf` parameter for restricting access to a share based on group membership.

Changing Unix file system group ownership (`chgrp`) and setting the GID bit affects underlying file permissions but does not directly control which users can establish a connection to the Samba share itself; `smb.conf` parameters manage share access.