The spell Rary's Telepathic Bond (PHB, 270; the SRD version of the spell is Telepathic Bond) allows targets to communicate telepathically for 1 hour, regardless of common languages or distance.
You forge a telepathic link among up to eight willing creatures of your choice within range, psychically linking each creature to all the others for the duration. Creatures with Intelligence scores of 2 or less aren’t affected by this spell.
Until the spell ends, the targets can communicate telepathically through the bond whether or not they have a common language. The communication is possible over any distance, though it can’t extend to other planes of existence.
Is it possible for someone/something other than the targets to hear this telepathy as well?
As an example, if a mind flayer was near one of the targets, would the illithid be able to hear all of the telepathic conversations, or just the outgoing messages sent by the target? Or none at all?
Best Answer
Elder Brains Can Do This
Elder brains are pretty much the end result of the most powerful illithids, so the ability to do so isn't particularly widespread, but per RAW it is the most concrete example I'm aware of that covers what you're looking for. They are discussed in greater detail in Volo's Guide to Monsters, but one of their Regional Effects is the ability to eavesdrop on any telepathic communications that occur within 5 miles of them.
In addition, you've the the Detect Thoughts option presented by Ryan, but that infers that eavesdropping is possible and is subject to some DM adjudication. This DM might give you only half the conversation if using that method.