The best way to get to know people is...
I've found that people tend to be more open and genuine in one-to-one conversations than they are when other people are around.
Add more people to the equation and people begin putting on a facade just to fit in and make a good impression, particularly if several people in the conversation know each other.
That's not to say it can't happen in a one-to-one conversation -- especially when both individuals met only recently -- but the chances of that happening are slimmer.
When it's just you and another person, you're more vulnerable. Efforts be silly in order to elicit laughs may blow up in your face, and there's no one else around to back you up in case you say something you may regret.
In addition, more participants bring more noise into the conversation. Suddenly you have people talking and shouting over another, not to mention going off on tangents. This makes it much more difficult to achieve the same degree of substance that a one-to-one conversation affords.
I find that the people I have the most superficial relationships with are those whom always seem to be around other people. Why? Because they don't open up the same way in that context.
Most people tend to act differently around one person than they would when their buddies are also present, for example. In this situation, they feel comfortable around their friends and generally make less of an effort to connect with the person they may not know as well.
If you want to get to know someone on a deeper level -- whether it's a date, friend of a friend, or co-worker -- arranging a one-to-one conversation is unequivocally the way to go.