Hi,
When you create custom groups, if you want to assign objects to these custom groups, you can follow the below steps to achieve it.
Assign objects to the custom group by creating shortcuts
1.Click the menu at the top of the Navigation Pane and, in the upper section of the menu, click your new category. The group or groups that you created for
your category appear in the lower section of the menu, along with the Unassigned Objects group.
2.Drag items from the Unassigned Objects group to a custom group. To move multiple items at once, hold down CTRL while you click the items, and then drag the
items to a custom group.
Here is a article about custom categories and groups in Navigation Pane that you can refer to.
http://office.microsoft.com/en-us/access-help/make-a-database-easier-to-use-by-customizing-and-controlling-the-navigation-pane-HA010234560.aspx#BM2
Best regards