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
@@ -1,5 +1,6 @@
 .\"
 .\" Copyright (C) 2001 Chad David <davidc@acns.ab.ca>. All rights reserved.
+.\" Copyright (C) 2023 Olivier Certner <olce.freebsd@certner.fr>
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
@@ -24,12 +25,12 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
 .\" DAMAGE.
 .\"
-.Dd July 9, 2001
+.Dd August 18, 2023
 .Dt GROUPMEMBER 9
 .Os
 .Sh NAME
 .Nm groupmember
-.Nd checks group set for a group ID
+.Nd checks if credentials mandate some group membership
 .Sh SYNOPSIS
 .In sys/param.h
 .In sys/ucred.h
@@ -38,21 +39,26 @@
 .Sh DESCRIPTION
 The
 .Fn groupmember
-function checks to see if the given
-.Fa gid
-is in the group set of the credentials.
+function checks if credentials
+.Fa cred
+indicate that the associated subject or object is a member of the group
+designated by the group ID
+.Fa gid .
 .Pp
-Its arguments are:
-.Bl -tag -width ".Fa cred"
-.It Fa gid
-The group ID to check for.
-.It Fa cred
-The credentials to search for the group in.
-.El
+Considered groups in
+.Fa cred
+are the effective and supplementary groups.
+The real group is not taken into account.
 .Sh RETURN VALUES
 If the
 .Fa gid
-is found, 1 is returned; otherwise, 0 is returned.
+is found, 1 is returned, otherwise 0.
+.Sh SEE ALSO
+.Xr getgroups 2
+.Xr setgroups 2
 .Sh AUTHORS
-This manual page was written by
-.An Chad David Aq Mt davidc@acns.ab.ca .
+This manual page was initially written by
+.An -nosplit
+.An Chad David Aq Mt davidc@acns.ab.ca
+and was revised by
+.An Olivier Certner Aq Mt olce.freebsd@certner.fr .