Add an example to pw.8 about how to add an existing user to a group.

Instead of using pw to modify group membership, users often edit
/etc/group by hand, which is discouraged.  Provide an example of
adding a user to the wheel group, which is a common use case.
I'm using a different user here as in the previous example as that
deleted the user (although the examples don't necessarily have to
be followed in order).

Reviewed by:	rgrimes,0mp
Approved by:	0mp
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D19123

usr.sbin/pw/pw.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 10, 2017
+.Dd February 8, 2019
 .Dt PW 8
 .Os
 .Sh NAME
@@ -979,6 +979,12 @@ Delete the gsmith user and their home directory, including contents.
 .Bd -literal -offset indent
 pw userdel -n gsmith -r
 .Ed
+.Pp
+Add the existing user jsmith to the wheel group,
+in addition to the other groups jsmith is already a member of.
+.Bd -literal -offset indent
+pw groupmod wheel -m jsmith
+.Ed
 .Sh EXIT STATUS
 The
 .Nm