Hi Shana,

The way I found to solve this is with a simple workflow rule built on the “Group Member” link. Here a few steps you could try:

1. Create Reference to Object  “User Group” field on User record. (You can set it as “read only” so it’s only updated by the workflow rule.) 
2. Create Workflow rule on “Group Member” link and have it run every time a record is created or edited. Group Member link can be found under “All Links > Membership > User Group Hierarchy > Group Member”
3. Set the evaluation criteria to: $Container.EntityType='UserGroup' && $Container.ResourcingGroup=TRUE && IsChanged($Primary) 
4. Set an Action to “Update Field” and have it update your custom field on the User record. In my example, my field name is “Primary Resourcing Group” so this action looks like this: $Member.C_PrimaryResourcingGroup. 
5. Set the formula in that action to “If($Primary=TRUE,$Container,null)”

In my testing, this clears the custom field if the toggle is turned off and set it when the toggle is turned on. This should be a good starting point.