If some resources don't work full-time for the company, the best way is to enter the agreed part-time schedule in the User's profile.

Please note that this setting is performed by the User (and not centrally).

To do so, click on the User's name / icon (top right corner) and then select "My Profile".

The daily parameters can then be entered in the "Schedule" section: 

Alternatively, especially for resources having irregular working times, you may simply book their unavailability by booking these slots as time-off.

Let's say you have a resource that's not available certain Tuesdays: simply create a project "Not available" (non-billable!) and book the appropriate days as per this resource schedule. If applicable, you can also make use of recurring bookings (say "every month").