diff --git a/tools/tools/git/git-arc.1 b/tools/tools/git/git-arc.1
--- a/tools/tools/git/git-arc.1
+++ b/tools/tools/git/git-arc.1
@@ -83,6 +83,8 @@
 .It Fl r Ar reviewer
 Add one or more reviewers, separated by commas, to revision(s) being created.
 Each argument must be an existing Phabricator user or group.
+Note that group reviewers must be specified using their
+hashtag (e.g. #jails for the "Jails" group).
 .It Fl s Ar subscriber
 Add one or more subscribers, separated by commas, to revision(s) being created.
 Each argument must be an existing Phabricator user or group.