As many people as you can gather can help look for it - yourself, your caddie, your mother-in-law, your hunting dog, your imaginary friend - but if your ball is not found within five minutes from the start of the search, you must take the penalty for a lost ball and move on.
Note, however, that the 5-minute time limit applies to the search, not to identifying your ball. Say someone who is helping with the search finds a ball within the five minutes, but it takes you a minute to get over to them and made the positive ID. That's OK. The ball was found inside of five minutes even though the identification fell beyond the time limit.
The time limit applies to all play conducted under the Rules of Golf, including rounds posted for handicap purposes. But in recreational play - a group of buddies out on the course, having fun, playing loose with the rules - you should never, ever use the full five minutes. Please, for the love of Tiger Woods, give up and move on so play isn't held up for everyone behind you.
