diff --git a/share/man/man9/Makefile b/share/man/man9/Makefile
--- a/share/man/man9/Makefile
+++ b/share/man/man9/Makefile
@@ -1157,6 +1157,7 @@
 	g_provider.9 g_error_provider.9 \
 	g_provider.9 g_new_providerf.9
 MLINKS+=gone_in.9 gone_in_dev.9
+MLINKS+=groupmember.9 realgroupmember.9
 MLINKS+=hash.9 hash32.9 \
 	hash.9 hash32_buf.9 \
 	hash.9 hash32_str.9 \
diff --git a/share/man/man9/groupmember.9 b/share/man/man9/groupmember.9
--- a/share/man/man9/groupmember.9
+++ b/share/man/man9/groupmember.9
@@ -36,6 +36,8 @@
 .In sys/ucred.h
 .Ft int
 .Fn groupmember "gid_t gid" "struct ucred *cred"
+.Ft int
+.Fn realgroupmember "gid_t gid" "struct ucred *cred"
 .Sh DESCRIPTION
 The
 .Fn groupmember
@@ -49,6 +51,11 @@
 .Fa cred
 are the effective and supplementary groups.
 The real group is not taken into account.
+.Pp
+Function
+.Fn realgroupmember
+works the same except that it considers instead the real and supplementary
+groups, and not the effective one.
 .Sh RETURN VALUES
 If the
 .Fa gid