Let's start with a preamble.

I don't like invading in most cases. It is too high risk. You stand to lose as much or more as you gain and you throw the entire jungle pathing out of whack so you and your jungler will have less of an idea of what pathing the enemy jungler will use.

But if you learn to setup properly to make yourself aware that an invade is occurring you will increase your likelihood of winning games. High percentage.

So this discussion is for everyone, not just junglers.

As soon as the game begins buy and be ready to run out of base as soon as the fountain wall falls at the 15 seconds mark.

As a jungler I ask my team in chat to cover the jungle

cover jg pls
Starting [Red or Blue] Buff

We are going to assume no special considerations like your Bot lane wanting to be in lane when the minion waves meet so that they can get a push verses the enemy lane or a Sion that wants to suicide on the enemies opposite buff.

If everyone goes where they are supposed to, it should look something like this.

Top laner cover the jungle entrance nearest to Top lane.
Mid laner covering the jungle entrance across from the river pixel brush
Jungler covering the jungle entrance nearest the Raptor camp, but behind the wall so they are out of vision
ADC and/or the support should go cover the tribush jungle entrance nearest Bot lane

Where do your wards go?

Top laner ward the scuttle crab around 120 before heading to lane.
Mid laner should hold their ward and see where the enemy jungler gets leashed before deciding where to ward
Jungler should hold their ward
Support or ADC should drop a ward in the pixel brush
Support or ADC in the tri-bush should drop a ward in the river that covers the tri-bush entrance before they come to leash (to prevent being surprised by the enemy Bot lane who will sometimes setup a camp in that bush)

If you get invaded when in this setup you should just drop a ward at the buff where the invade is occurring and simply fade back into your jungle.

All you would prefer to have is a headcount on the invaders.

-If the ward shows a 5 man invade and them staying to steal your buff. Don't panic.

The jungler and your Top laner should go to their opposite buff and take it.

Mid and Bot should go to their respective lanes.

As jungler you will probably want to Smite the camp to make sure your Top laner can get to lane as fast as possible. Since you stole their Red buff it may be a perfect time to go gank their Top laner from behind as they will probably have no idea you are behind them.

You can then look for the Top Scuttle crab and then look for a gank in Mid or go back to farming your Top side jungle.

-If the ward shows a 4 man invade one of two things happened.

Their Top/Mid laner is AFK at their tower
Their Top/Mid laner is hanging out at their opposite buff to prevent if from behind stolen or possibly they warded it before going to lane

This presents a dilemma.

The high percentage play is to have your Top laner leash you on your opposite buff and you just write off the lost buff.

And if you got a ward down before fading away you will have the timer on your stolen buff, which presents an opportunity for laying at trap around the 7 minute mark when the enemy jungler comes and tries to collect your buff again.

The lower percentage play is to go check on their opposite buff. It is lower percentage because if the enemy Top or Mid laner are there they can waste your time and your Top laner's time and time is not a thing you can really afford to waste in the early game.

Reality

Bot lane will go to stand beside your buff about 90% of the time because they have been trained that they are responsible for leashing. 10% of the time they will run straight to their turret and you will have to ping them over to you.

Mid lane will AFK at their turret 75% of the time. 15% of the time they will come stand with you the jungler. And 10% of the time they will go guard the bush across from the Top side pixel brush.

Top lane will 75% of the time AFK at their turret. 25% of the time they will go stand at the Top side entrance to your jungle.

Worst case you run and ward the pixel brush on the Bot side of the map. Then you run over to stand in the intersection just below Blue buff because this will let you see both paths and run away if you get invaded.

75% of the time your Bot lane will think you are starting Blue even though you said you would start Red and they will head to lane. Ping them back over.

You need to leave about 1-1:10 to make it over to Red from the intersection below Blue buff.

So even if your team does nothing to help you guard your jungle you will by your own efforts have the jungle as covered as you possibly can.