Sounds like a neighborhood watch group of sorts, if I had to guess.
GMRS is going to be far more expensive if the 20 users are all separate and not family members. $70x20=$1400. You also get the pleasure of dealing with the wasteland that is GMRS, and all the garbage that goes with it. You are also forced to use analog, as digital is not allowed on GMRS.
Assuming this is for something "business" related, the Part 90 license makes so much more sense. Frequency coordination and the license fee will likely be around $500-600, which is going to save you money, potentially, and you don't have to deal with GMRS, the kids, roger beeps, music, people screaming, people playing television and radio shows, people re-broadcasting their scanner, etc. If it's anything professional, GMRS should not even be in the list of options.