You'll need team of 4 people, 3 just won't cut it.
All 4 must be at level cap of 25, with good equip and knowledge how to use it.
If one of your friends doesn't has it - just grind Normal a few times, he'll get there. No need to drag him into overwhelming fight.
Naturally you can create conference chat in Steam for better coordination, but it works even without it, as long as all team member watch each other on map and perform appropriate roles

There are 3 roles:

Driller

His task is simply keep the spinning up the drill all the time each 30 secinds, even if less than 30 seconds of wave left.
Between spin-ups he can use placed MGs and personal firearms to place a few shots at horde nearby.
Probably the easiest task, only true danger are trackers, they are moving extremely fast and can slip past defenders easily.

Jungler

His task is to go to side-missions during the waves and along with crates look for rainbow-colored blobs of Iris energy, each of them adds 5K to the Iris amount mined.
After finishing side-missions he joins defenders.

Defenders

Two of them fight the horde, using all the gadgets placed inbetween waves, doing their best.
Naturally, this is the hardest and the most dangerous role, better to be taken by the most able to fight head-on team members.

It's quite simple:

1. After deployment gather resources, place drill ASAP to get minimap working, but do not start it prematurely.
2. Make sure any player close will get side-mission done before start of the wave.
3. Whole team places traps and turrets along the lanes to make defenders' job easier.
4. When wave starts, driller must immmediately start spinning up the drill, keeping at it the whole wave. He will have around 20-25 seconds to run to the wall and place a few shots at distant enemies or run-in tracker, but his main focus is the drill
5. Jungler departs to hunt for the side-missions to grab those precious Iris blobs.
6. Defenders take their places to cover up lanes, keeping the horde in check.
7. Jungler finishes side-missions and returns to rovide backup to defenders.
8. After wave ends - go to #2
9. Enjoy your hard-earned S-rank :)