Hi EllaKopella
Sounds to me like you have a similar role to me. I'm the PM for our IT projects. However, I also have some senior user responsibilities as I work for the IT Helpdesk and will be supporting the projects products. On top of this I'm often supplying kit and installation services, so have Senior Supplier services too. In effect, I'm doing some of the 'grunt work' for my own projects!
However for defining the project scope, we have tended to have 'brainstorming' sessions with users to discuss what we believe should be included. We also make sure we specify any exclusions from the scope of the project. Then a case of sitting down the project sponsor, Steering group and agreeing what's 'in' and is not. The fundamental here though, is that product requirements should be heavily driven by the user representation on the steering group.
Always a good idea to ensure that scope is managed through its lifecycle, though. Scope creep can happen and if it does the impact of the changes need to be fully assessed rather than just quietly added.
Good luck.