The idea is not yet a startup. The startup is a project. The project has a product or service, resources, and processes. Your idea does not have this all yet, but it is a perfect moment to start giving a shape to your startup.
Brainstorming and collecting ideas from founders, friends, and mentors is a good enough beginning but not yet a result. You have to get rid of all additional features, define the root functionality, find out what you are missing and of course make a market investigation. This last thing should be not only there, but do it on every step ever and ever! If you have no eye on market or people doing that - leave the idea.
Now you have the goal, core functions, business idea and probably got the vision how your product might evolve. Plus you know who of the founders is strong in what - it is time to make it happen! MVP!