For the first week or two of my freshman year of college I would sit down next to random girls and spark up a conversation. I ended up getting pretty good at it too. This one time I sat down at a table and just started talking to this group. Eventually we introduced ourselves and the girls were surprised and impressed that I didn't know anybody there. I sat down with so much confidence that they thought I was one of them.
After the first few weeks of doing this I had enough numbers that I really didnt need to do it anymore(especially since its not the best way to find hot girls). As soon as you number close a decent up kept looking girl text her. Ask about hanging out. Odds are she is going to want to bring her friends. BOOM you just met a group of smoking hot girls with little effort