@GL L.
If they are established investors in the market, I personally wouldn't be that concerned with them trying to "steal my deal". I'd be upfront with them in that I'm currently negotiating on it, and I want them to have the first shot at it. Also, let them know you're in the game for the long haul and look forward to this being the first of a long string of deals you'll bring them.
Serious investors won't steal your deals.
That being said, if you don't have a deal yet, they won't like it that you're wasting their time looking at a house that isn't even tied down yet. If I were you I might give them the basic details of the deal so they can do a quick preliminary, instead of having them spend their time visiting the property. That will also give you an idea of what price you need to be at. But use this strategy sparingly, because it's still using their time for something that may not happen.
In any case: do what you can do get the contract signed ASAP---you can't sell a deal you don't have.
