Doesn anyone know what exaclty this power does. How much of someones' mind can you see through this? Seems like its somehting that has to be mutual right? Is this mostly for comunication, so you can talk to party memebers without npc's knowing?

I would say you can create a link to send information according to the power scale under the telepathy discipline. This send and receive would be concious action so you would not 'see' accidentally another's thoughts. Useful for silent communication, and basic visuals at higher power.

I know it kinda sounds silly, but for me its like, you have skype conversation with some friends and your the host. The more powerfull the psyker becomes they can also see what the other people see?

Just as you described OP. Involentary probing is covered by a different power.

It's a good idea to discuss how much information is being sent. I allow messages and some visual information, but draw the line at the notion that the PCs can all see what every other PC is seeing at once. The sheer amount of information would overload the mind's ability to process it, at least in a timely fashion. Not good in time crucial situations such as combat.