devtrackers.gg
Dealing damage isn't the only way to grab aggro, I've played enough tank to know what things play into the aggro algorithm. Proximity, line of sight, duration that you've had aggro, being in melee range, all can allow a tank to hold aggro. If you're the first person to shoot a shot in a room, and you don't ever lose line of sight on a target, you'll be able to hold that aggro well. I've held boss aggro in a crowd of 4 people all within 5 meters of him, because I had already been holding his aggro for a good while while my team cleared the room.
If you aren't careful, you can lose aggro, and it'll be very hard to get it back without walking up to them and giving them a slap, but if you know what you're doing, you can keep your team bullet-hole free!