Watch the Building a New Groove online guitar lesson by Fareed Haque from Jazz Rock Workshop
In this example, I'll hunt around for a third guitar part. Look for the holes. In any groove, not doubling, but finding a part that slips in between the existing parts will usually be the funkier choice.
You can create that contrast, or the space for your part, by contrasting not only rhythms, but also finding a contrasting tone, attack and register. For example, if all the guitar parts are low, then play something higher up on the neck. If the parts are all wah-wah parts, then don't add another wah-wah. If the other guitars are distorted, use a clean sound. If you are playing with a rhodes, clavinet, or b3 organ, try to find parts that fit in the holes and have a different sound than the other instruments in the track.
Have fun and don't be afraid to experiment.