So, I am just beginning a BC campaign where the players are trying to do a ritual to summon the pieces of Erebus. Its a grand task for people new players, but with the help of Tzeentch they will likely succeed. What I am wondering though, is through this journey they will be chancing insanity by looking into the warp, so I would like to know, how would you describe the warp? How would you describe its many demons? (mostly how they interact with real space on there own terms. Like cults and such.) And how easy is it to get possessed or to resist possession?

That's all in the core book is it not?

To be fair a lot of this is probably about our perceptions or a fluffy based question so I shall answer based on my interpretations of the warp:

The warp itself can be seen as a twisting maelstrom of colour but also ideas, concepts, terrors and delight. The right mind staring into it to divine could see what might, what will be, what is, but also what will never be or use it to sway the words of warriors, warlords and shaman while to the same token decimate men, armies and legions

Warp rifts like what you will be showing them may also contain brief glimmers of the realms of each gods or the wastelands between them so seeing the vast foundries and war factories of Khorne, the twisting labyrinthine archives of Tzeentch, temptations and desires of Slaanesh or the embrace of decay that is Nurgle. Even this though is enough to send a mortal insane or at least hurl oodles of corruption points their way. Few can stand near a warp rift unprotected without some consequence. Be aware though that as some might look into the warp through such tears, other figures look out through them at reality beyond.

Concerning daemons, many are the creations of their patrons and in some way represent elements of their persona and physical images. Khornate creatures therefore often resemble vast hulking snarling warriors of bull headed appearance or bestial monstrosities. Nurglings meanwhile often as diseased bloated figures but quite jovial in their nature. Every daemon though is just a representation of their gods personality at the time of its creation...unless its an unaligned daemon and little is said of those in current fluff so go nuts.

In terms of daemonic interaction with realspace, this is limited. Daemons cannot usually maintain their presence in our world in the same way that a mortal could not survive long in theirs. After a time in our world, unless managing to bind themselves and anchor themselves to our realm they will be soon cast back. The same is not true of humans on their side though. In order to pass through there must be a weakness such as an untrained psyker for a strong daemon to posses and use to their own ends, a warp rift which creatures of the immaterium may flow though. There are however ways that stronger daemons may remain in our world for prolongued periods of time, usually by tainting the world or maintaining by the blood and souls of innocents.

Finally, resisting possession, that's all in the core book on how easy it is to resist. I refer you to the Possession rules on P142 of the BC Core Rulebook explaining the act, the effect, the consequences etc. And there's one of the books which has a right of expelling them (Tome of Decay I think, haven't got it to hand to remember the page).