Added documentation for getgroups policy function

Ticket: ENT-12722
Signed-off-by: Victor Moene <victor.moene@northern.tech>

Author: victormlg

content/reference/functions/getgroups.markdown

@@ -0,0 +1,47 @@
+---
+layout: default
+title: getgroups
+---
+
+{{< CFEngine_function_prototype(exclude_names, exclude_ids) >}}
+
+**Description:** Returns a list of all groups defined, except those names in the comma separated string of `exclude_names` and the comma separated string of uids in `exclude_ids`
+
+{{< CFEngine_function_attributes(exclude_names, exclude_ids) >}}
+
+**Example:**
+
+```
+bundle agent main
+{
+  vars:
+    "groups"
+      slist => getgroups("root", "");
+
+    "first_three_groups"
+      slist => sublist("@(groups)", head, 3);
+
+  reports:
+    "$(first_three_groups)";
+
+}
+```
+
+**Output:**
+
+```
+R: daemon
+R: bin
+R: sys
+```
+
+**Notes:**
+
+- This function is currently only available on Unix-like systems.
+- This function will return both local and remote (for example, groups defined in an external directory like LDAP) groups on a system.
+
+**History:**
+
+- Introduced in CFEngine 3.27.
+
+**See also:** [`getgroupinfo()`][getgroupinfo].