If there are jobs or work that you are not yet sure who the best team member is to assign to or you have a teammate leaving your firm or going on leave, use a placeholder user to hold the place of the assignees till you have a replacement.
Create a new colleague called "1 - Placeholder User" or similar
You can use a fake email address for that new colleague
As long as the email invitation is never accepted, the new colleague will not be added to your account's number of users, and you will not be charged
You can still assign any work item to that ""1 - Placeholder User" user