You need to do it gradually and be prepared for a lot of chasing & teeth chattering! I have a group of girls and then when one dies I'll add another one or a pair. The way I do it, is first of all a quarantine period of a few days, just to make sure you're not bringing in any disease. Then put her/them in an adjoining cage or split the pen so they can sniff each other through the wire. Then I have them on my lap together so they get used to each other (most guineas won't have a fight on your lap!). Then I usually swap the new ones with the old ones in their living areas so they all get used to each others smell. By doing this they should all end up smelling pretty much the same if they are using each other's bedding. You can put them on neutral ground with a big pile of nice food, will take their minds off each other. A lot of people on here bath them so they smell the same, that's a good idea too. It's a gradual process. I'm going through this again at the moment with mine. I'm at the very eary stages! Sometimes it's taken me one day, another time it took me 3 weeks to introduce a particuarly dominant female.
Good luck! ps you need a good amount of space for them all. I'm lucky in that mine have got large pens. Don't think it would work so well with small hutches.