Disguise does require a successful roll to gain experience, so it's hard to build up from scratch; it's usually easier to have people train it solo or in small groups, because each person has to make an independent check and if anyone fails, your entire squad's cover is blown and nobody gets experience. All you need to do to do a disguise check is just run into a group and try to walk away from them - it's automatic. Bluffing is only available if your cover has already been blown. You can also raise disguise by sending people to learn theatre at the university, or have someone who already has a few points in the skill teach infiltration.