Okay, so how do you feel left out? You must first define why you feel this way. Is it because your group of friends keep talking about stuff you don't get or is it because they don't give you a chance to say anything? There could be other reasons but I'm only going to talk about these two since they are the simplest and the most likely reasons.
So if you feel left out because you don't know what's going on, then just wait for them to talk it out first. When the conversation pauses a bit, interrupt them with something that you know about or ask them to explain what they're talking about. When you interrupt, don't be in the back and say something in a weak voice. Come forward and project your voice loudly. Say what you want to say without hesitation and don't be flamboyant about it. You have to be somewhat stern if you want to grab their attention. Dominance and a little bit of harshness can actually take you places. As long as it's not too much and you know how to use it, people will pay attention to you.
If you're never given a chance to talk, then, like I suggested for the previous one, come out with what you have to say strongly and sternly. Don't be shy about it. They're your friends, you should be comfortable enough around them. If they still overlap you, then it's okay to shout every once in a while about your opinions. They might go "there's no need to shout" and you can reply back saying "well give me a chance to speak and maybe I wouldn't have to give myself a chance to shout" or something like that. Just make sure that your voice is heard and that people are focusing on you. You need to establish your presence in the group before people will actually take you seriously.
Just keep in mind that being serious, dominant, and stern doesn't mean that you are mean. You just need to be assertive about what you want is all. There is no need for insulting people (unless they actually deserve it) or getting into fights. You just need to throw yourself out there and stand up for yourself.
Anyway, if you have any more questions, feel free to ask me. Hope this helps.