You could add an extra entity called 'projectrole' to do this and link this entity to a project as well as an employee!.
Furthermore, check out Rom's remark, your assocation projects_employees now says that a project only has 1 employee, you might want to turn this the other way around if you wish, or make it a many * - to - many * association!
Also very important: please keep your association lines straight to improve readability :)
Your domain model should look something like this:
Check also this lecture: https://gettingstarted.mendixcloud.com/link/module/123/lecture/1009
and this one: https://gettingstarted.mendixcloud.com/link/module/123/lecture/1080
Or start at the beginning; https://gettingstarted.mendixcloud.com/link/path/38
p.s. please use singular names for your entities; you will thank me later ;-)