The Group Labels command is used to group together labels on distribution equipment, feeders, and equipment connections on the one-line diagram. Labels in a group can be modified all at once using the Move Label and Rotate Label commands and the Alignment and Scale columns of the Insert and Modify Labels dialog box.
To group labels together, go to
Ribbon: DM Electrical->One-Line Diagram-> 
Group Labels
Pulldown Menu: DM Electrical->One-Line Diagram->Group Labels
You will be prompted to select a label, which will be positioned at the top of the group.
Select label in correct location:
You can then select one or more labels, including other groups of labels, to add to the group.