connect Track calls to Groups

Author: spencerattick

src/connections/destinations/catalog/mixpanel/index.md

@@ -126,6 +126,8 @@ Mixpanel supports multiple definitions of groups. For more information see [Mixp
 
 If the group call **does not** have a group trait that matches the Group Identifier Traits setting, then the event will be ignored.
 
+If you'd like to connect a Track call with a Group in Mixpanel, simply send the Group's ID as a property on the Track call named `$group_id`. 
+
 ### Group using Device-mode
 
 When you use Analytics.js, you must set a `userId` value, or Mixpanel will ignore Group calls.