You can log time for other users if you have the security permissions to do so. For instance, managers or supervisors can be granted permissions to log time for their team.
To enter time for another user, a vendor or an employee, you need to disable the permission 'Allow access only to entries of projects I manage' from the Security Permissions screen. Enabling this option allows you to access the time entries of only those projects that you manage. So after un-checking this option, you can see and enter time for other users. This can be done by any person having access to the Security screen.