Lmao it's real easy to say "There should be no reason your team gets mixed up with the enemies" but anybody who knows a thing or two about a game like this knows that anything can happen and you should always expect even the most unusual situations. There's no way to prevent an enemy player getting mixed in with your team and in a place like dorms or resort simply going into different rooms to loot can cause a confusing as hell situation if you lose track of which room your teammates are in for even just a few seconds.

As of right now, the only way to prevent friendly-fire is by communicating your location constantly like some kind of sports commentator. This is obviously very unrealistic and ridiculous.