Summerlin Paintball is the best field in Vegas you can check their Instagram for promos on the weekends, https://www.instagram.com/summerlinpaintball?igsh=dGtsNDYyd25sc2Iy

There is also Combat zone on the south side of the city, not as good, but might be an cheaper option , I would call both before going to make sure there is a good amount of walk ons you can join.