After months of stealing other pros' drills, today I have one that I made up spur-of-the-moment yesterday. Great warm-up for any size group, any ability. No special equipment needed. Yes, there is the dreaded 'line' of players waiting to hit, but this goes quickly.
Players line up at service line or baseline, however you prefer to warm up. Coach feeds balls to players from net post area. If player hits safely, he/she goes to end of line. However if he/she hits out or into net, ALL players run to opposite end of court, re-form line, and continue activity. Repeat for every miss. Yes this means Coach needs to switch sides also so be prepared. Coach may feed wherever they want to adjust the challenge accordingly. Go for as long as you want or until players are sufficiently warmed up.