How do I assign users to clients and tasks?

Assuming you have set all your client information correctly, most user assignment should already be complete.

You can separately assign users directly to clients in the Companies or Individuals sections, by clicking on the client name, and updating the internal assignment panel - Account Manager field

You can assign or reassign users to individual tasks via the Tasks window, by selecting the specific task and updating the internal assignment panel - Assigned To field

By selecting a client in the Companies or Individuals tabs you can set a service manager for a particular service using the Assign button. The account manager retains overall responsibility for the client with the service manager responsible only for the specifically selected service.