You just have to shift your perspective of things
Right now people don't believe shared dream is possible, because they think is something that goes on inside each individual mind that is enclosed in each individual head.
But I ask you: how is it possible that we share "reality"? It is, like dreams, just information that your brain processes and you perceive in a certain way. But reality isn't really real! There is no solid, material, external world if there's no conscience to make it real. All there is "out there" is clouds of possibilities and potential, energy and information waiting that our collective conscience decides what to make of it. All of the reality was we know it is our common, shared dream. And this is no joke. Physics is going on that direction but old philosophy and ancient wisdom tells exactly that. Check buddhism for instance - it is all based on this fact and it gives you the tools to train your mind to perceive that clearly.
Dreams are just a more subtle level of mind and shared consciousness. Just as in "reality" we can feel ourselves as the centre of the world and miss the point that all we experience is projected by all of us together, when dreaming we can have dreams that are just ours because we don't realise the matrix in which they are embedded is actually also projected by us all. When we do open ourselves to the fact that consciouness in all levels is one and is embedded in this one matrix, then we start sharing the information that is available on this shared matrix.
It's not like we are antennas sending out our dreams like radio shows. If it helps to see it that way, that's ok, but what really happens is that minds are not separated, therefore they don't really need to send out anything for another mind to capture - it's all about opening our perception to the fact that my mind is not separated from your mind and that we can in fact "meet" at the same "place" at the same "time" on this dream-level matrix, just as we meet people outside on the street - be it a random encounter or an appointment.