Excuse the spelling, but are any other Psijiccs who make an appearance other than Divayth?
And to my knowledge they are a religious cult who embrace change, but there is more to them than that

The http://www.imperial-library.info/content/guide-psijic-order is a good place to start. It'll also give you the names of the most prominent individuals, including Divayth, Sotha Sil, Loremaster Celarus, Vanus Galerion and Mannimarco (both ex-members), Urvwen - you can probably gather some more names from 2920 (or at least names of students back during the First Era). Granted, the only ones who "make an appearance" (as in, in-game) are Fyr, Sotha Sil (if you call that an appearance), and Mannimarco.

Calling them a religion is a misnomer, they're more akin to philosopher-mages who act as diplomatic counselors. Not really sure what the right word for them would be.

right word? alumni of the same college.

Mixed in with a little jedi consular. Minus the lightsaber.

And to my knowledge they are a religious cult who embrace change

Perhaps the sage Taheritae said it best: “In Mundus, conflict and disparity are what bring change, and change is the most sacred of the Eleven Forces. Change is the force without focus or origin. It is the duty of the disciplined Psijic [“Enlightened One”] to dilute change where it brings greed, gluttony, sloth, ignorance, prejudice, cruelty... [here Taheritae lists the rest of the 111 Prodigalities], and to encourage change where it brings excellence, beauty, happiness, and enlightenment.

He also made an appearance in post #2 of this thread.

Dude gets around fo sho.

I swear that was not there this morning :wacko:

you don't send prominent and respected members of your order to Black Marsh.

What is the relationship between the Psijic Order and the Psijic Endeavor?
Nothing but a namesake, I am sorry to say. Now we can end that mystery. The Endeavor is a method of achieving the Tower and then what to do after. The Order is not, and does not really care for the idea of the Endeavor at all, thinking it wrong-headed nearly from the start.
So what is the Psijic Order?
A monastic order of the Altmer, impressive with age, and obsessed with a kind of mythic genealogy, made famous by their monomythism. Theirs was mostly the study of reverse-ancestry as a means to combat spiritual and material degradation. Or at least it was, I should say, as I do not know what they do now and do not much care. They were wrong-headed nearly from the start.
It is interesting to note that their original views were very unorthodox for Altmer, and thus their exile from Alinor. These views included the suggestion that Anu’s son, the Time Dragon, was formed in reaction to Padhome’s influence. In effect, Anu had finally done something. This inconceivable effect gave rise to an equally inconceivable cause, and so PSJJJJ was named and the Order eventually took his name.
Veloth was not a member of these exiles, no. Some of the Order’s precepts and ideas do (or did) flourish in the beginnings of the Alessian Order, the Temple of the One in Cyrodiil, most Maruhkati sects (including the Selectives, though they would deny it), Ancestral-Mothism, and, most famous of all, the Mages Guild.