Yes, add the folks that you want to log time on behalf of as External Resources. In the Admin zone select Resources>External Resources>Create/Import External Resources. They look just like regular users but they can not log in and therefore do not take up a seat/license . You may opt to have them receive emails as well.
Once External Resources are in the system you can then add them to projects and assign tasks to them. As the PM you may update a task and select which External Resource you are logging time as.
You may also update time via the timesheet but a permission on the back end must be checked - I will post up how to do so later today but that should get you going for now.