Why do you not go sit with them instead of going off and sitting alone? Why do they have to ask you rather than you go be with them if that is what you want? Think about it from their point of view; if you were someone (you are :-) and had to go "beg" someone to come sit with you or you were with one or two other girls and all sat down, how could you get up and go get this other girl and have her come over, etc.?

We cannot be "seen" unless we at least move/make ourselves and our presence known; that's our job, not someone else's to see us.