Question on custom field and pulling in users based on a group.

ESM, Service desk, creating custom field that "looks up" data in a SW Group?

I have a group called Dept Managers and in that group I've added only those that are managers.

I want to create a custom field that will look up or display the users in this list to pick from.

I then want to add this custom field to the catalog item form, and have the requestor pick a user from this list.

In the process task to send a notification to the user that is in this field.

This custom field will work if i pick the field type of  "User"  but it looks up all Users.  I want to filter this to only the users that are in the group.