I'd add either another female or a neutered male.
Steps I'd personally use (keep in mind I've only introduced females to males for 4-H, which is a pretty straight forward process, some one else on here is likely to know more):
-Quarantine. Done for any new pet. Make sure you aren't bringing in diseases/parasites.
-Set cages next to each other. Let guinea pigs smell/see/hear each other for two or three days.
-Switch the cages they are in so the new one can get a smell of the old ones, and the old ones can get a good smell of the new one. Switch them a few times over the course of two to three days.
-Introduce guinea pigs (maybe one of your old girls at a time) to the new pig on neutral ground. Somewhere they do not have their scent on. Offer treats and hiding places.
-Supervised time together in a spacious cage (the more room, the less you usually have to worry about fighting) with hiding areas provided.
-Move them in together permanently if all is going well. If things are not going smoothly, start back on square one and slowly work your way up again.
Here is how someone else does it:
http://jackiesguineapiggies.com/Companionship.html