Right, I'm assuming you're talking about approaching people in generally social places, such as the pub / club scenario.
In that case, you might want to read up a bit on body-language... if you know what to look for, there are general queues people will give off which gives a better indication on whether or not a group of people would be OK with another person joining their group . For example, if two people are talking, and are facing each other at an angle (and possibly looking around occasionally), they're (generally) more receptive of a third party joining their conversation than if they're facing each other dead-on and they clearly have each others full attention.
On a night out, say, the girls who are most receptive to talking to people tend to be those in groups of 2-3. It's perhaps best if you have a friend (e.g. a wingman) to talk to the other girl(s) while you concentrate more on the one you're interested with. Having said that you'd need to show some attention / consideration to her friend, as her "
bessie" has to approve you, otherwise she'll perform the "c**k-block" manoeuvre. If you have to talk to them both without a mate, then keeping something like a 70:30 ratio of attention is a good bet. Popular conversation topics can be how they know each other, why they're out etc.
The other thing to do is to make sure they can see you approaching them. That way, if you spot an "Oh no, he's coming over" type look, you can dodge them at the last minute and save any awkward interactions.
Avoid very large groups of girls like the plague (unless you're a stripper lol). The large group of girls on the p**s may seem like rich pickings, but it often results in abuse and humiliation (just take my word for it
). If you're feeling exceptionally brave (or suicidal lol), it's best to approach one girl when she's away from everyone (e.g. going to the bar etc.)... the mistake people do here, is they'll go for the most drunk girl in the group. Remember her less intoxicated mates will be watching her like a hawk and woe-betide anyone who's considering taking advantage of that.