....and I just answered one of my own questions. To add a new label to an existing label group, type the new label into a blank table cell where the label group is applied and the new label will be added to the group. You may also apply a label to a page and when choosing a different label at the page level, "add label" is an option there.
I have also discovered that a label group can be transferred to a new page if you apply the label group to a page (not a table column), select the label element on the page, then copy and paste it to a new page. The title and options of that label are now available on the new page. This has to be done one label group at a time it appears.