As a coach, you train one or more teams participating in the Scrub Cup. Your job is to get them to work as a team - most players will only be used to Solo Queue ranked. Additionally, all participating players are 2500 SR and below, which means they are less used to comboing ults, engaging and retreating as a unit etc…
Because of that, you will have to teach them things that seem totally obvious to you - don’t trickle in, don’t give them free ult charge, suicide when the fight is lost, combo ults… You will need a lot of patience and be willing to think outside the box - strategies that work at Diamond and Masters will not necessarily work in Gold and Silver, while some comps that seem totally ridiculous to you might actually be viable.
Help players to work as a team
Work on strategies
Work on mentality
Help with morale
Coaching players individually (of course possible, but in no way expected)
Micromanaging your team
Shotcalling (contact during a game is forbidden!)
Don’t be condescending! The participants aren’t idiots or bad at Overwatch because they want to, but mostly because they just can’t put the time in it takes to become good or because this is their first FPS. Speak with them as equals and consider their feedback and wishes.
Give feedback properly! Basically, don’t say “you did this wrong, and this wrong, and this wrong.” It makes people defensive and less likely to follow your advice. Be honest but kind with your criticism. For example, lead with something positive - “That earthshatter was great! However, did you notice that it left your team open to a counter shatter?” Having people point out their OWN mistakes (not those of their teammates) is also good - if they are aware of it they’ll be happy to take advice on it.
Get your team scrimming and record it! Many of your players will never have seen a VoD of themselves playing, just seeing yourself play for the first time will make many mistakes obvious without the coach having to say anything.
Watch VoDs together with your players! Mayhem.gg allows you to watch videos together with your team and review them collaboratively. FlowFeedback allows you to annotate VoDs at specific times and makes your tips much more useful. This is much more engaging than posting a wall of text in the team discord, and will be much more effective! It’s specifically designed to make coaching in Overwatch easier.
Get someone to shotcall. Moving decisively together is always better than everybody doing their own thing, even if some calls are bad. You might not have someone with calling experience on the team and no one volunteering, but don’t just assign one - have the team decide. You can firmly point them in the direction of the designated Lucio player. Don’t try to make your caller micromanage the team, they’ll have their hands full with basic calls.
Use your player’s strengths - you might have some OTPs, try to accomodate them as best as possible instead of forcing a comp. You won’t have the time to teach them a couple new heroes.
Focus on one or two compositions your team does well instead of one for each situation. Dive comps are fun, but take a lot of coordination to execute well. Having a solid core with for example Reinhardt, Ana, Lucio, Diva and then flexing on the DPS slots will work for almost all maps and gives you enough time to perfect the comp.
Consider non-meta strategies - Pharah is very strong if the opposing team has no good hitscan to reliably take her out. You need to know how to counter her and you need to consider using her.
The same goes for a number of other strategies, such as Bastion or Symmetra-Torb defenses.
Watch your team’s morale too - calm them down when things get heated, cheer them up after a lost game, praise them. Praise is a great tool to keep spirits up, use it regularly but only when it’s true - don’t overdo it and don’t praise if they don’t deserve it - they’ll know.