diff --git a/util/groupmember/groupmember.go b/util/groupmember/groupmember.go
index 2a9b5d24e..3597bfa76 100644
--- a/util/groupmember/groupmember.go
+++ b/util/groupmember/groupmember.go
@@ -7,14 +7,28 @@
 package groupmember
 
 import (
-	"errors"
-	"runtime"
+	"os/user"
 )
 
-var ErrNotImplemented = errors.New("not implemented for GOOS=" + runtime.GOOS)
-
 // IsMemberOfGroup reports whether the provided user is a member of
 // the provided system group.
 func IsMemberOfGroup(group, userName string) (bool, error) {
-	return isMemberOfGroup(group, userName)
+	u, err := user.Lookup(userName)
+	if err != nil {
+		return false, err
+	}
+	ugids, err := u.GroupIds()
+	if err != nil {
+		return false, err
+	}
+	g, err := user.LookupGroup(group)
+	if err != nil {
+		return false, err
+	}
+	for _, ugid := range ugids {
+		if g.Gid == ugid {
+			return true, nil
+		}
+	}
+	return false, nil
 }
diff --git a/util/groupmember/groupmember_cgo.go b/util/groupmember/groupmember_cgo.go
deleted file mode 100644
index 008a62987..000000000
--- a/util/groupmember/groupmember_cgo.go
+++ /dev/null
@@ -1,49 +0,0 @@
-// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build cgo && !osusergo
-// +build cgo,!osusergo
-
-package groupmember
-
-import (
-	"os/user"
-	"sync"
-)
-
-func isMemberOfGroup(group, name string) (bool, error) {
-	u, err := user.Lookup(name)
-	if err != nil {
-		return false, err
-	}
-	ugids, err := u.GroupIds()
-	if err != nil {
-		return false, err
-	}
-	gid, err := getGroupID(group)
-	if err != nil {
-		return false, err
-	}
-	for _, ugid := range ugids {
-		if gid == ugid {
-			return true, nil
-		}
-	}
-	return false, nil
-}
-
-var groupIDCache sync.Map // of string
-
-func getGroupID(groupName string) (string, error) {
-	s, ok := groupIDCache.Load(groupName)
-	if ok {
-		return s.(string), nil
-	}
-	g, err := user.LookupGroup(groupName)
-	if err != nil {
-		return "", err
-	}
-	groupIDCache.Store(groupName, g.Gid)
-	return g.Gid, nil
-}
diff --git a/util/groupmember/groupmember_noimpl.go b/util/groupmember/groupmember_noimpl.go
deleted file mode 100644
index 5a0d9a061..000000000
--- a/util/groupmember/groupmember_noimpl.go
+++ /dev/null
@@ -1,10 +0,0 @@
-// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !cgo && !linux && !darwin
-// +build !cgo,!linux,!darwin
-
-package groupmember
-
-func isMemberOfGroup(group, name string) (bool, error) { return false, ErrNotImplemented }
diff --git a/util/groupmember/groupmember_notcgo.go b/util/groupmember/groupmember_notcgo.go
deleted file mode 100644
index a2102701c..000000000
--- a/util/groupmember/groupmember_notcgo.go
+++ /dev/null
@@ -1,72 +0,0 @@
-// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build (!cgo || osusergo) && (linux || darwin)
-// +build !cgo osusergo
-// +build linux darwin
-
-package groupmember
-
-import (
-	"bufio"
-	"fmt"
-	"os"
-	"os/exec"
-	"strings"
-
-	"go4.org/mem"
-	"tailscale.com/version/distro"
-)
-
-func isMemberOfGroup(group, name string) (bool, error) {
-	if distro.Get() == distro.Synology {
-		return isMemberOfGroupEtcGroup(group, name)
-	}
-	cmd := exec.Command("/usr/bin/env", "groups", name)
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		return false, err
-	}
-	groups := strings.Split(strings.TrimSpace(string(out)), " ")
-	for _, g := range groups {
-		if g == group {
-			return true, nil
-		}
-	}
-	return false, nil
-}
-
-func isMemberOfGroupEtcGroup(group, name string) (bool, error) {
-	f, err := os.Open("/etc/group")
-	if err != nil {
-		return false, err
-	}
-	defer f.Close()
-	s := bufio.NewScanner(f)
-	var agLine string
-	for s.Scan() {
-		if !mem.HasPrefix(mem.B(s.Bytes()), mem.S(fmt.Sprintf("%s:", group))) {
-			continue
-		}
-		agLine = s.Text()
-		break
-	}
-	if err := s.Err(); err != nil {
-		return false, err
-	}
-	if agLine == "" {
-		return false, fmt.Errorf("admin group not defined")
-	}
-	agEntry := strings.Split(agLine, ":")
-	if len(agEntry) < 4 {
-		return false, fmt.Errorf("malformed admin group entry")
-	}
-	agMembers := agEntry[3]
-	for _, m := range strings.Split(agMembers, ",") {
-		if m == name {
-			return true, nil
-		}
-	}
-	return false, nil
-}