It sounds to me like you have a joint venture. A joint venture does not have to be in writing to be binding. If you are associated with two or more persons in an enterprise that is a joint venture (which is under partnership law). You were approached with an idea. A business plan was partially drafted (apparently with your involvement). It sounds like you did something in relation to this proposal and it is clear you want to pursue an enterprise. The fact that the other persons are not pursuing this as you like does not mean that the venture was dissolved or you are free to take something you admit was not yours.
So I think this is under partnership law and you are a fiduciary. That means you cannot just go and take assets from this venture for your own benefit. Do so and you may be sued (you may not if they are not smart enough to speak with a business attorney).
Instead of create problems you should just agree to dissolve the partnership and allow each partner to pursue this idea if they want to. This dissolution and agreement should be in writing (see above as to why). Good luck and speak with a local business attorney.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
Attorney Murillo is correct; it seems as though the arrangement between you and the other people has advanced beyond 'idle chat over a drink or two' and has progressed to the point where a partnership may well have been created, despite the same not being formalized in writing / with the State. IF that is the case, then your attempts to snatch the idea may well subject you to suit. Of course, there are many variables at play that may NOT subject you to the same; say, if the business venture idea was so overly basic ("I bet a pizza place on this street would make a fortune!") or the like. Your best bet is to sit down with an attorney; many provide free initial consultations, myself included.