Can you get 3 adults,one child and 3 suitcases in a yellow cab?

Yes, unless suitcases are so large they will not fit in the trunk. If you are arriving at JFK the rate from the airport to Manhattan is fixed at $30. Does not include a tip or toll if required.

Yes to the number of people. Someone may have to sit in the front. You should be able to get 3 suit cases in unless they're very large.

Yes to the number of people. Someone may have to sit in the front. You should be able to get 3 suit cases in unless they're very large.

As long as we're talking about taxi's, how does cab sharing work? What if you're not going to the same address? How do you split the cost?

You may be able to find a yellow cab that is a minivan. They're out there (I get a minivan yellow cab maybe 1 out of 50 cab rides), although you probably can't count on finding one when you need it.

As for cabsharing, for regular cab rides in NYC, taxis will let people share rides and drop people off at different places. The person who gets out last is responsible for the final bill, so that person has to work out a deal with the person(s) dropped off first.

For rides from JFK airport, the set fare of $30 applies to the first drop-off in Manhattan. The taxi driver will probably then start the meter for passengers getting out at other locations.