If you've been playing guitar at least a little while there's a chance you've already been playing an add9 chord. There's a certain chord which shows up in everything from Southern Rock, to Power Ballads to Pop and Hard Rock. You know your open G chord on guitar, the version where you include your pinky on the thin strings? Now, you know when you move your two top fingers down to create a version of a C chord? Guess what. That's a Cadd9 chord! Some songs even include that version and the standard open C Major chord in the same song. It's a great way to get expressive and sounds get whether you're playing acoustic or electric.
The problem is, that open Cadd9 shape isn't easily movable. If you're more advanced on guitar and can handle bar chords you can play a movable add9 chord. Check it out in the diagram. The numbering is the suggested fingering and the root note is the '1' note. The root note here is an E note (7th fret, A string) so you should be able to work out how to move this barre chord to form other versions for your guitar playing.
Some great guitarists have used versions of this movable shape from ambient and spacey guitar work to shredders. You can use this chord the same way you use any chord - in other words, you can strum this chord or come up with a cool picking pattern.
You can vary this chord by barring the whole fret where you first finger is. You'll get new sounds doing this when including either the high 'B' note or low 'B' note (both on 7th fret, high and low strings). Incorporate these add9 chords into your guitar playing. You could use this with some generic Major open chords, or you might like to try some barred minor or minor 7th chords. For a more tense sound try something augmented.
Finally, these chords can be pretty demanding to play. Give your fretting hand a rest if you need to.