I'm going to provide some advice on the thread for people who show up to my smashfests. You don't have to take the advice, and you're welcome to voice your discontent with any of the advice I give. I also apologize if this post, or anything within, comes off as condescending or patronizing. In the future I hope to post something of a guide for general matchup strategies (more of a "vs. this character" type guide than a "playing as this character" one).
Samir - Mostly, you need to play safer. Marth's aerials are safe only insofar as they are spaced properly. You also need to juggle more: against Sheik, it's standard fare to just juggle by means of uthrow, uair and utilt. You'll get a hang for doing this well by practicing. Against Fox and Falco, you generally have a similar pattern, except
1) On Final Destination, you stick to uthrow regrabs (which occasionally involve uairs and utilts, depending on percents; I recommend watching videos of M2K doing it to learn it properly, and I think he's written a guide) and try to end in a kill or by hitting the opponent off the stage and edge guarding.
2) On the remaining neutral stages, save for Pokemon Stadium and Fountain of Dreams, you try to make use of the low platforms, for otherwise the space animal can attempt to survive the juggle by landing on the high platform. When they are on the platform above you, you need to be attentive to when and how the opponent has landed on the platform and attack accordingly; attacking too fast or too slow will result in a whiff and the spacey will drop through the platform and attack you.
3) On Pokemon Stadium, chain grab as though it's Final Destination save for when you're already underneath one of the short platforms.
4) On Fountain of Dreams, chain grab as though it's Battlefield or Yoshi's Story, but be less reliant on the chain grab, for it does not work as consistently with the platforms changing altitude.
These can be applied in similar fashion to Sheik, but eventually you want to compartmentalize juggle mechanics for each opponent, since they all have unique weights, falling speed and acceleration. In general, keep in mind that you are strongest when your opponent is above you and weakest when they are below you (save for when you're edgeguarding).
Rohan - I didn't get to see you play much. From what I saw, you need to move a lot more. Don't just dash dance in place in response to an opponent dash dancing. Dash dancing is used to keep your position changing while simultaneously allowing you to act; if you're dash dancing with too much distance between you and your opponent, it won't serve a purpose. As you play more, you should become more familiar and fluid with Marth's movement and movements, which will allow you to combo better and play better overall. In the mean time, try and have some idea as to what combos and approaches "work" and try to emulate them. You'll most likely go through a trial and error in which you correct these things as you go. This won't help you attain any sort of idealistic "correct" style of play, but it will help you improve. Once you've reached a certain skill (this level of skill being subjective and up to the individual), you can start to optimize your gameplay from a more theoretical standpoint, addressing more technical questions.
Jamal - You tend to do all right when your opponent approaches badly, but otherwise you struggle. In general, this means that you don't know how to combat camping and that you don't know how to react to "good" approaches. Study up on Sheik's individual match ups. You should also grab more, and be getting much harsher combos out of your grabs. Make better use of your needles as well.
In the future we'll record matches and I'll try to upload them to my Youtube channel. I can't host a smash fest next Saturday because of my friend's wedding (getting married on 4th of July weekend ftl) or the weekend after because I'm going to Houston. Hopefully the weekend after, or some random Friday or Sunday, I will be able to host something.
If there's anything else I can help anyone with, let me know.