At this point, you should have a list of stakeholders with their roles and contact information. Now it's time to make use of it: be sure to make time to talk with each stakeholder about what they need from the system and how they envision it. You can hold interviews such as 1:1 meetings or group ones. When talking with your stakeholders, help them to make informed decisions – show the potential outcomes of their answers on the end product.
It's common for stakeholders to say that all of their requirements are equally important. Try to persuade them to prioritize their requirements according to the value they bring to their business. Certainly, there will be some mission-critical requirements, but most probably, the project won't fail if a bunch of others won't be delivered, not to mention any nice-to-haves that will land on your requirements wish list.
Aside from interviews, you can also organize workshops for...