By default, all users can enter time to all active projects provided you have the relevant subscription package and security permissions. There are only a few things that would prevent you from doing this:

1. Employee Control has been implemented and that project is not assigned to you.
2. Project Assignments have been implemented and you are not assigned to that project. 
3. The 'Prevent time entries' rule has been applied to that project. In this case, no one can enter time to the project.