When the other members of the group are friends with each other, it's much harder to get a word in edgewise because they're used to talking back and forth between themselves and simply don't 'make space' for another voice. Or if one member makes him/herself the 'leader' of the group with a strong personality, he/she will essentially control the discussion and who participates in it. It's unfortunate that our difficulty in social skills occasionally hurts our academic performance, but in situations like these, it does. My only advice is to just keep trying. A good idea is a good idea, no matter who says it. Hopefully, when you have one, they'll listen.
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Everything would be better if you were in charge.