I believe there was a Nextel phone years ago that had a built in phone to phone digital frequency. No expert on this, but looking at the gotenna page I guess what it does is put you on 2 meters vhf with your phone. I got some murs radios which are 2 meter plus my handheld ham radio is also 2 meter. If there was a cheap add on for cell phones that offered 2 meter that would be great.
Edited to add....the original gotenna apparently operates on a 2 meter MURS frequency. My murs radios will give you about a 2 or 3 mile range. You can get a bag of four gotenna for about 240. Don't know how they work for a group chat. I think for emergency prep, at this point, having some regular radios might work better.