- Who is our target market? Who are we building the chat app for?
- How do we get traction in this market?
- Who are our competitors? What is our differentiator / unfair advantage over them?
- What are the monetization plans?
- What is the end goal? Being acquired by a big company, IPO, lifestyle business etc
- What are the responsibilities for each of the three founders? How would each of them add value to the business?
- How would we know if we are on the right path? e.g. x% per month increase in number of users/messages/revenue etc
At the end of this, you will either be sold on the idea or you will not. I'd advise you to join only if the two guys are able to convince you of the idea.