To determine who your users are answer these two questions:
- What is your PURPOSE? (i.e. what does your team/organisation aim to achieve?)
- WHY should anyone else care about your MISSION? (i.e. what's in it for them?)
Write down all the groups you can think of in answer to question 2 (along with the reason they should care about what you're trying to achieve) and start focusing on them, as these are your users.