Having the ability to merge contact groups would save time by eliminating the need to click through each profile to add/remove a group or a multi-select system for contact groups lists

Assuming it works as it used to, try this:

Copy and paste the entire contact list into a Google sheet (including all rows of the table the contacts sit within).

Select only the column of character names from Google sheets.

Copy them.

Go to the contact list you wish to put them in and paste it into the "add contact" box. 

Click add.

It may look messy but it should add the entire list to the contact group.

But, good suggestion. +1