Hi! After reading your post, I believe it isn't necessary for you to ask your friends to join the other group. If you are afraid of having only one group remaining, rest assured that Group Ninja War will still continue and will still give you Group Ninja Packs even if there is only one group participating.

As well, if your server has decreased in players and you believe a merge might be helpful, I highly recommend submitting a request here.

~Jib