Redmine

Hello, I would like to ask for advise what is the correct procedure for following redmine enhancement.
I want to create a form with option select where I will pick the user and assign him to an existed project with under certain role. The form can be part of existed or new site (file.html.erb).
I am new in redmine. I know some basic scripts in ruby and basic folders' structure in RoR.
I know that Redmine & RoR works with databases' tables through models. The data are displayed on site (file.html.erb) based on scripts in controller.
I need to acces data in two or three tables to make the form functional but I do not know whether I need to create a new model/table which will be associated with required tables or there is option to access these tables somehow directly => it means to add or update data in various tables through existed/new controller. Pls help to advise what is the best-simple way for form implementation.

Thanks for your time!
Regards
Adrián