Telepathy: Telepathy (or rather, techlepathy) is a somewhat limited faculty, as described under Psychic Powers. Being essentially a way to transmit neural gestalt information over WiFi, it is an excellent messaging system, complete with the ability to transmit images, emotions, information constructs, and other non-verbal information, and communicate broadcast/narrowcast, local/distant, private/conference, etc., etc., and even to synch up well enough to form a conflux (see: Mental Fusion).
It doesn’t, however, let you overhear people’s thoughts (unless they’re sloppy and accidentally broadcasting, which does sometimes happen), go in and pull thoughts out of their heads (you want a SQUID for that, or a mind-state reader of one sort or another), control minds (although you can inject the same memes as you can over any communication channel), or enable the rest of the assorted magical aids and attachments (dream travel, astral projection, and so on and so forth). It’s also, being EM based, limited to the speed of light.
Since it’s harder to fake all the sidebands, a telepath can sometimes be something of a Living Lie Detector, but reliability in this role still isn’t great, and you are almost certainly better off with a regular mechanical alethiometer (which uses a SQUID).
Hive Mind: The Transcend is not a traditional Hive Mind, it’s a collective consciousness, or group intellect. The difference, as it will point out to you at some considerable length, is that all of the members retain their individual personalities and perspectives as well as contributing to the whole, that while knowledge is shared, individual memories aren’t (at least until one is poured into the soul-ocean on permadeath), and that the true hive-mindedness is that of the Transcendent soul-shard that’s grafted into your personal consciousness (see Touched By Vorlons), but which does not constitute all of it. And that the Transcend has a (very large) metamind of its own (see Deus Est Machina), over and above and simultaneously within the collective aspect.
There are also, of course, genuine Hive Minds which do abolish individual identity, which are called Fusions, but since (a) resistance is not futile, because they only take volunteers, and (b) by far the most common type of Fusion is the Self-Fusion, made up of multiple instances of the same mind, they’re not anything to worry about.
(Okay, yes, in civilized space. Hegemonizing swarms with a degree of intelligence are not unknown outside it, even to the extent of things like the Leviathan Consciousness, and the less said about things like the Calphrast Hordes the better, but still…)
The temporary kind (see the Mental Fusion trope) are known as confluxes, often used for tactical military purposes and in other situations where close teamwork is required.
Mental Fusion: See Hive Mind for the permanent version (Fusions and Self-Fusions).
The temporary version, called a conflux, is a standard part of the arsenal for a variety of situations – merged fireteams for greater military efficiency and coordination (and, y’know, battlefield omniscience, when done really right), guard webs that share knowledge and raise the alarm simultaneously, research teams thinking in perfect coordination in which the brainpower of the whole is greater than the sum of its parts, and so on and so forth. It takes a lot of attention paid to mental discipline and interpersonal relations to build a team that can do it well, but when it works, it really works.