Quoted from s1500:
Also, if a game is in progress(first ball), and not all the players have gone yet(but the first player has), don't ask to join in, add your quarters & hit start. Some games completely reset and thus the first ball scores are long gone. This happened once on a Devil's Dare machine, costing me.
If they're playing a tournament game, absolutely. But if it's a practice game and still first ball, I'll always invite others to join in. It's common pinball courtesy. Pretty much every game built in the last 20 years will allow you to add another player without resetting everyone's score.
Whenever possible, I'll ask if others want to join in. Playing on location, at home, wherever. I recommend others do the same. It won't earn you any karma points here, but it will in real life.