The Cairn are a very advanced telepathic species who have lost the ability to communicate verbally. They send telepathic images instead of words, which allows them to send information at a vastly faster rate to other telepaths. However, other telepathic species cannot cope with image telepathy unless they have mastered the mode themselves, so even telepathic species have trouble with the Cairn.
As the Cairn wish to join the Federation, they have begun using vocal enhancers to help them talk with other species. Lwaxana Troi was assisting them in 2370 but suffered consequences of using the unfamiliar telepathic mode, in the episode “Dark Page“.
The Cairn have difficulty understanding the concepts of privacy and omission.
Beta canon says they achieved membership by 2378. You can likely have a Cairn join Starfleet from 2370 onward, either accepted into Starfleet Academy, or as an officer on exchange through the Officer Exchange Program.
