The technical key to playing funk guitar is in your fretting hand. Although most of the chords and riffs you'll play will be simple, you'll need to learn to deaden strings with your fretting hand, to create a rhythmic sound. Rarely in funk music is a guitar chord allowed to ring, as it is in pop/rock music. Rather, the note or chord is struck, then almost immediately deadened, via releasing the pressure on the string(s) with the fretting hand. Practice this technique with various chords. Of course, the picking hand is also very important. Strings should be played firmly, with great attention to rhythmic detail.
The role of the guitarist in funk music is quite different from pop music. The funk guitarist's job within the band is mainly to be rhythmic, and will probably have a lower profile then he/she may be used to. Often, a funk guitarist will repeat one simple rhythmic figure for five minutes at a time, without variation. Guitarists looking for the spotlight on stage often don't make great funk musicians. Great discipline is necessary.
Give the Drummer Some
Your role as a funk guitarist is essentially the same as the role of the drummer. It's not about notes - it's about how you fit in with the rest of the band rhythmically. Turn your attention to the drummer, and focus intensely on what he is doing. Concentrate on making what you are playing "groove" with what the drummer is doing. If you can lock with a drummer, you can bet you'll be called first when other musicians are looking for funk guitarists to play with.