Demons are bound to space. They are not omnipresent (Job 1:7). As long as demons have no legal rights, they cannot not share space with us and hence, cannot access our minds.
Demons communicate with each other by pure thoughts while we humans do the same by physical means such as tongue, paper, gesticulation, electronic communication etc.
Even if demons are not mappend onto us, they may inject thoughts into our minds (John 13:2). We are free to accept or reject these thoughts. Once accepted, they produce (negative or positive) emotions until we act according to the demons' voice against God's will, giving the demons legal rights to map onto us. At this stage, the demons are able to read (at least partially) our minds, to further explore our weakness until we act according to the demons' personality.
A practical example is telepathy within the field of parapsychology where thoughts are transferred from one person to another. In this case, at least two demons are involved. One demon, mapped onto the sending person, continuously reads the transmitter's mind and broadcasts the thoughts. The other, mapped onto the receiving person, collects and transfers this information to the receiver's mind.