If I used TRX only, including cardio, would I still be able to put on muscle mass, or would I just lean out?

 It's the same as with weights. It depends how you use it. Use heavy resistance, bulk up; low resistance high reps and get lean.

 So, when you say heavy resistance, do you mean including a weighted vest while using TRX? Is that safe?

 It's safe to wear a vest, but i meant harder progressions. Like the incline press, or moving away from the anchor point for suspended pushups.

Thanks, I am now getting a better understanding. A weighted vest definitely helps though, right?

 It could, but you really don't need to use one until you master exercises at their hardest progressions with the proper use you can do most exercises at 100% of bodyweight. So if you're say 200lb the deep chest press with elevated feet would be like doing a bench press with two 100lb dumbells (because of the lack of instability with a barbell it is a lot easier than with dumbells).  If you can already do that then you'd need the weight vest.