I usually don't feel bad about doing whatever the heck I wanna do on my own time, so I just tell them I do...well..what you said, a lot, but that I am bored and that I would like to hang out with "you lot"....You lot being the friends you just befriended.
If you feel guilty about making your own choices in life, yeah, they're gonna burn you for it, but if you're just honest, and say that's what ya do, and you're up for what they're up for(And if you're not, just say it, and explain why).....They'll either accept or not, it's better then duplicity. Duplicity leads to drama. Drama leads to soap opera, soap opera leads to evil twins, psychoses, split personalities and unwanted pregnancy. Don't lie:P