If Raven brings them together to stop Trigon, there should be no question or ambiguity about it. Either she's recruited and only reveals the truth when she trusts them, or she lays her cards on the table from the word go.
Though frankly, I'm kind of tired of Trigon. Every Teen Titans incarnation tends to repeat those story elements and either just reach the same heights (the cartoon) or drastically undershoot them and waste everyone's time (the New 52). Plus, he's much less fun than the fanatical Brother Blood or the awesome Deathstroke. He's an evil demon lord. Of evil. Sadly, those are a dime-a-dozen now.
Having said that, here's how I'd do Raven. 18 year old Raven is recruited by Nightwing when he finds her being hunted by Brother Blood's acolytes, just like I suggested in the quick pitch on page one of this thread. She's recruited because of the de-facto arms race going on over meta-humans, and joins Cyborg and Nightwing's team. She's got telekinetic and teleportation powers like in the cartoon, and like in the cartoon, these powers rely on her excercising maximum self-control. She was on the run with a family that raised her because they know about Trigon, and it's heavily implied that Nightwing's investigations make him privy to her parentage and her wariness of reaching her 23rd birthday. She kind of knows that as well, and they have a very sibling like relationship.
Raven's personality is formed by her empathy/emotion eater capabilities, her knowledge and fear her father is going to try and use her to escape on her 23rd birthday, and her conflicting love of life. As a result, Raven is both very passionate, adventurous, and curious, but wary of hedonism and of wasting time. She's got patience because she has to have it to manage her emotions and their powers, but it's clearly a daily struggle that she only succeeds at because she's practiced for years. And it's unspoken, but Raven has determined that unless someone can immediately stop Trigon the moment he emerges, it would be better for everyone if she didn't make it to her 23rd birthday. Thus, why she's seemingly so morose and despondent while always being attracted to upbeat friends and acquaintances; she's not suicidal and doesn't really have a death wish, but is resigned to the idea that it's going to have to happen.
Then, after the first adventure (maybe the second if a series took off) she finally trusts everyone enough to tell them what's going on, and instead of Trigon escaping, they enter Raven's spirit world to free her and defeat her father.