First of all welcome, glad the game is still getting fresh blood. (not talking about level since that is mostly meaningless I mean players that are clearly new to the game or are still learning the mechanics/controls) 

That out of the way I am noticing people have obviously not watched many videos on the game and the game's tutorials are not the greatest for some interactions. Also some general etiquette that is being breached.

1: Using a stim when at full or nearly full health will remove your mortal wound status

2: Stims heal whatever amount they are supposed to for the difficulty IN ADDTION TO RESTORING ALL CURRENT CONTESTED HEALTH, if no enemy is nearby to execute to get that health back don't be afraid to pop a stim to fully restore if you just took a big hit towards the end of a fight.

3: If you have a Bulwark that is level 23 or higher and have noticed him giving you health back, save your stims. You communicate with him to put a banner down for you when you are on mortal wound and can then stim back to full and remove your wound in one go.

4: As you may have noticed stims are important and the Bulwark amplifies their power, if you have one DO NOT just instantly stim as soon as you pick up. Again ask your Bulwark if he can banner you before using that stim when you are on very low HP.

On to the etiquette

5: Please, do not use Ruthless as a means of leveling faster if you are just starting and have not really learned the basics. You are forcing others to pick up your slack. I am not even necessarily talking about going down, I mean you just are not doing enough damage with your under-geared weapons. 

6: Pop open the team list (TAB default on k/m don't know what it is on controller) and check what other's stim supply is before picking up stims UNLESS the other two have already pinged the stim in which case they are signaling they do not need/want it.

7: If a teammate is on mortal wounds and stims are in short supply/you don't have a Bulwark to heal them, let them pick up the guardian relic since that will save them from death if they go down before managing to clear that wound.

8: Geneseed should be carried by whoever seems like the least likely to go down. Failing that, anyone who still needs experience. That said, plenty of veterans are willing to carry it so always ping the geneseed if you find it or ask in chat (politely) if someone else can carry it if you are not confident. There are plenty of players who like to roleplay and "feel honored" to carry it. 
8a: Bonus on the geneseed - If someone is on mortal wound due to bad luck, they can still carry the geneseed if they feel like they are not going down again since the geneseed is lost on down anyway. Oh and you can swap guardian relic and geneseed so if you want to swap, letting the carrier pick up the relic means now they don't have to carry it any more and someone else can.

Mechanics:

Only one in this category and that is the gun on Termination. You have to AIM at the bio titan (you will see the three guns have laser pointers to indicate where they are aimed) and then HOLD DOWN the fire button to shoot. You get three shots, it is okay to miss but please just shoot. You are actually holding up the game if you pick it up then just fight things, leave the heavy lifting to your two brothers and know that under the walkways is safe from the cannon fire.

If I have missed anything please feel free to add more. 

A lot of excellent comments!

Shining_Darkness wrote:
Pop open the team list (TAB default on k/m don't know what it is on controller) and check what other's stim supply is before picking up stims UNLESS the other two have already pinged the stim in which case they are signaling they do not need/want it.

In addition to this, I would add that the same goes for ammo. It is important to consider the classes of your teammates, if there is a Heavy Marine, offer the ammo before taking it. If you take it all yourself, you incapacitate a teammate. Same with armor boosters, offer it if you don't need it!

________

Shining_Darkness wrote:
you just are not doing enough damage with your under-geared weapons

I have to say that I failed at this one until I realized my mistake. When leveling up my last remaining class, the Heavy Marine, I started playing on Substantial difficulty while being at Level 1. I was doing less than 7K damage while my teammates were doing 15K or 16K, which was obviously a problem for them.