Previous releases used a group entry beginning with a `+'
(plus sign) or `-' (minus sign) to selectively incorporate
entries from a naming service source (for example, an NIS
map or data from an LDAP server) for group. If still
required, this is supported by specifying group:compat in
nsswitch.conf(4). The compat source might not be supported
in future releases. A possible sources is files followed by
ldap. This has the effect of incorporating information from
an LDAP server after the group file.