@Strannger18 said:

I guessed at first it was the gas used to put them into a dream state. But they repeatedly say that it only for sleep. So how do they cross the divide.

How? Well technically they are breaking into someone's dream so they are not sharing it per se. The main dreamer has a slight advantage.

It's the machine they use. It uses a computer to generate a virtual dream space (constructed by the architect) and then imposes it on the minds of multiple sleepers who are hooked up to the machine.

Here's a picture of Arthur and Ariadne sharing a dream: https://vignette.wikia.nocookie.net/inception/images/2/2e/Dream_sharing.png/revision/latest?cb=20131124021334

There is a device which is used to share the dream. Remember, the architect programs this dream using multiple levels. They are not entering the dreamers head. There are all entering a shared dream using this device which has an architected dream. The dreamer only populates this dream with his or her subconscious. It's this device that you will see them use everywhere. One a train, in the plane. The device is used inside the dream as well to go into further levels of the per-designed dream. That is how they are able to share the same dream.

@tivep said:

Here's a picture of Arthur and Ariadne sharing a dream: https://vignette.wikia.nocookie.net/inception/images/2/2e/Dream_sharing.png/revision/latest?cb=20131124021334

There is a device which is used to share the dream. Remember, the architect programs this dream using multiple levels. They are not entering the dreamers head. There are all entering a shared dream using this device which has an architected dream. The dreamer only populates this dream with his or her subconscious. It's this device that you will see them use everywhere. One a train, in the plane. The device is used inside the dream as well to go into further levels of the per-designed dream. That is how they are able to share the same dream.

Good explanation. Thanks.