You have a decision if you would like to borrow their pads... I would still wear a sweatshirt just in case but if your tough you can go with a muscle shirt and shorts and wear no pads, that's what I did. Wear a cup also, because last time I didn't, and I really regretted it... :D
When I played I used to wear a padded long sleeve undershirt along with my body armor. I used to play real competitive so you may not need all that. If you get shot at close range it will sting and leave a bruise.
When i played they charged extra for body armour but had a ton of sweatshirts you could borrow for free, so i just went with those......it stings but i had an adrenaline rush the whole time so didnt feel much of anything...my teammate(girl) accidentally shot me in the back of my head, I was kinda ughed about it, but i didnt let it stop me from playing ( ;-D love paintballin!!!!! Such a fun thing to do when youve got the right gang ;))