That is a pretty open ended question. For a small group of recreational divers that just dive a couple of dives per day, one of the little 3-4 cfm portable gasoline compressors work just fine. It takes about 20 minutes to fill up a tank so with a group of five of you , you are looking about an hour and a half. That is fine for a little down time between dives.
As for your group of 20, the compressor depends on what your group does. If you are a rescue team, and you do several training dives at a time, then you will need a portable to bring to the dive site. For that size of a group, you will need to have 7 CFM or bigger.