I think it would be just as easy to disband one (or more) group(s) and have all the interested members join the other (remaining) group. I think you have to get chess.com to "delete" a group once it's no longer needed or wanted.
You can click on Help & Support at the bottom of the page and contact the staff with your suggestion.
I just thought of an idea.
There are a lot of groups on chess.com that invite identical target demography, such as the groups that invite only the member of a certain country, religion, fanclub, etc.
Is there a way to merge similar groups into one supergroup? If not, could that be a possible addition to the features of this wonderful chess site?