I know it might seem hard but you need to try going up and speaking to people - you will, in 99% of the time find them to be friendly and approachable. I usually make a point of trying to say hello to people I see at either of our events (in my sig) if they appear to be on their own or if I don't recognise them, and I then make sure that when I go off, I have introduced them to at least 3 others so they are not 'on their own'. Not everyone appreciates being spoken to but I, and the people I do the nights with, do try and do this anyway because we genuinely welcome and want to encourage new people to come along, regardless of whether they are 20 or 70.
If the crowds seem to be clicky it's because they are most likely friends who have been meeting each other at various nights for many years. Doesn't mean to say that anyone is unwelcome. The fact is, if there are 15 people at a night and they all know each other (and this IS likely these days) then of course you are going to feel like an outsider but it doesn't mean that those other people see you as such, and you can either go home and cry about it or do something about it. If it was either of our nights, I will personally buy you a pint and force you to speak to at least 6 people at the next one (attendances allowing ). If it wasn't then, as said, sort out a meet up with some others who are going either there or beforehand and you won't look back.....
Martin