When your wild sorcerer rolls a 69 on their bullshit table, everybody laughs and then everybody turns invisible. It’s a whole situation.
What does invisibility mean mechanically?
Invisibility works like reverse blindness. If you’re blinded, you have disadvantage on attack rolls and attack rolls against you have advantage. When you turn invisible, everyone is blinded to you.
Invisibility DOES NOT mean you’re undetectable. Your enemies in combat still know where you are and can target you. You’re still leaving footprints and whooping loudly like a silly bird. But…
Invisibility breaks line of sight. So you can take the hide action to become undetectable. You normally need to duck around a corner or behind something to break line of sight and attempt to hide. [Sage Advice reference.]
Invisible creatures can’t be targeted by opportunity attacks because opportunity attacks require you to see the target. [Sage Advice reference.]
What do advantage and disadvantage mean mechanically?
Advantage is good because it makes you roll 2D20 and keep the higher.
Disadvantage is bad because it makes you roll 2D20 and keep the lower.
But they cancel each other. No matter how many sources of advantage or disadvantage are in play, you just roll a single D20 as normal. [Sage Advice reference.]
So how should mass invisibility play out?
Everybody knows where everybody is unless someone takes the hide action (in which case it’s a stealth check vs passive perception).
As soon as you attack, cast a spell, or take a major action at DM discretion, you lose invisibility.
If you’re invisible, your attacks have advantage.
If you’re attacking an invisible target, your attacks have disadvantage.
If you’re both invisible, you roll as normal.
What’s the best strategy when everybody’s invisible?
Hold your action until somebody become visible. Remember that you only get a single attack when you prepare an action to attack, regardless of whether you have the Extra Attack feature.
Take the hide action to potentially become undetectable. Use it as an opportunity to get into a better position before kicking off again.