How can i show interest?

i work at a local park as park patrol. i drive a gator(pretty much a 4 wheeler/golf cart) around and make sure the parks r safe. im 19 and had a prime opportunity to get somewhere with a group of 3 attractive girls. me and my co worker/friend drove past them purposely because we knew they were taking pictures and my friend figured they would need someone to take pictures for them. they asked us to take a pic and my friend said he had a bad knee (which he did, he had a crutch) so he let me take the pics. i took 2 pics and got a little joke in there but as soon as they got what they wanted i gave back the camera and got back in my gator. i guess i froze up, i was a little nervous, but if they woulda started talking to me, i woulda been ok, i just couldnt strike up a convo. what should i have done?

but a better question is what should i message this girl? my friend thought of the idea of me sending a friend request on facebook to one of the girls. he is already friends with her because we went to school together, she also recognized him and maybe even me before she asked us to take the pics. so he thought when she accepts the friend request, i should message her asking her if her or her friends dropped a ring because we found one where they were. then after she says no, i should to try to show interest in her and her friends. after she says no, what should i say?