Rachel's dream aside, the rest was him hijacking TV or Radio to implant in them the idea they should go to Elmo or whatever it was... Rachel's dream happened on its own IMO, as in she sensed she had to go to Elmo, as that is where Titans would be meeting. Or Bruce got a telepath or something to put the idea in her brain. Of course it was all a bit too dramatic and one does wonder he could probably simply call like a normal human being and set up a meeting with them the normal way. Doubt they would decline.
The problem for me is, Donna and Dawn do not have the knowledge we have about Dick's and Gar's situations. All they know is Dick is locked up in prison, which may be his own doing, but Rachel is seeing dreams of him dying -killed by Deathstroke no less!- so he is possibly in imminent danger that he is not aware of. And that Gar and Superboy are missing and need to be rescued. Bruce just gave them a speech on how important it is for them to be together and work as a team and pretty much told them to rescue Dick (pretty sure he was doing his magic with the diner TV). Donna, even before Bruce's speech, was desperately trying to reach Dick as she knew his help was much needed to find the lost the boys and rescue them. So, as far as they know, Dick is in danger and in need of help AND Gar & Superboy are in danger and in need of help. Of the two, they know exactly where Dick is, and have good reason to think it is Deathstroke going after him to kill him. So, going to his rescue as a team would take what, a few extra hours? And with Dick back in the fold, their chances of finding Superboy and Gar would be increasing. Instead they abandoned him once again, split from the team to go look for Superboy and Gar on their own. In doing so, they pretty much abandoned Rachel and Kory on their own to deal with Deathstroke -and they didn't even know Kory was going to help out, it very much seemed like it was going to be Rachel on her own!- If there is one thing these people should know, it is you never ever face Deathstroke just on your own... So, it wasn't a carefully thought strategic decision, it was a decision made with a "he wants to rot in jail, fine by me!" attitude, just because they are upset with him over Jericho and the fact he got himself in jail in a stupid effort to pay for his "mistakes", a state of mind they very much had a hand in putting him in, btw. They have a problem with Dick's self-hating and self-punishing, but don't have a problem with putting all the blame on him all the time, completely ignoring their part of it all. It bugs me on so many levels. Worst friends ever, if they can be called that.