My friends are leaving me out, what do i do?

My friend, lets call her Brooklyn, has recently made friends with another girl, lets call her Ellie. Brooklyn and Ellie always go of and leave me on my own. I try really hard to be good friends with them but they always go off and when I try to join in with their conversations, I don't have a clue what they are talking about. They have names for all the people they fancy but I don't know who they are talking about. Before, when Ellie wasn't friends with Brooklyn, Brooklyn used to go off with another girl, we'll call her Amy, and they would do similar things but now Brooklyn is friends with Ellie, Amy and I fell left out all of the time. I don'y know what to do and it is getting quite upsetting now... :(