This is an odd request but I've noticed a number of series and oneshots where a central character is "left out". For instance, you have three dear friends, and one friend firmly believes in their bond and that they'll always be together, or at least that they'll always be open with each other and not do anything without everyone's knowledge and understanding. Then the person finds the other two going at it, possibly talking either about how they're fooling their friend or how hard it is to reveal that they're fooling this friend. Then there're also stories where you have several people doing it together, but one person is in some way "left out". Like the person's just there as an extra or a tool for the others' pleasure. For instance, a work I stumbled across a while ago had a guy's little sisters repeatedly propositioning him; he hated it because he felt they didn't really care about him but they'd push and deny it until he let in, but once they started he basically amounted to a toy with which they could pleasure each other, with both giving all their love and attention to each other and at most giving brief lip service to their big brother. There are even a few where a guy will find that his girl(s) are more interested in toys than him, and even if they go back and comfort him they still quickly forget that and continue to play with the toys instead. There's other variations, but the core is always the same: someone is in some way left out, either excluded completely or ignored despite being included, and incredibly frustrated/depressed on account of this. This happens with both guys and girls. And every time I want to see the person decide (s)he's not going to stand for such treatment. Therefore, please let me know if you know any good stories where a person is left out or ignored in this way but decides to take action and gain full inclusion. Even if this requires the person to be a little aggressive.

Also fine with stories where the others themselves realize that the person in question is left out and legitimately include him/her, as long as this is a full and lasting inclusion.

Not really a genre I like but you could try Shiawase Shoujo chapters 6-9 (When You Let Go My Hand), not sure if it's what you search since the MC start the manga at the moment he learn the truth.
Or maybe School Girl (chapters 1-2 "The Man I And My Mom Love") but again not sure if it's what you are searching. Maybe not enought left out ?

I loved When You Let Go My Hand when I read it some time ago, despite me generally being turned off by such massive chests, because I liked the story. While it is certainly a type I'm happy to accept here, I wouldn't say it fits either because the guy wasn't trying to not be "left out", rather he was trying to save the girl from the situation she was in. Here I'm looking more for someone who's either forcing his/her way in and taking someone for him/herself or jumping in and insisting on being a part of the group (if it's a group thing or has some implication of togetherness, like one of three friends finding him/herself left out and not even informed of the other two's relationship).