You can add new users on demand, there is no problem with that. When new users are added, if they belong to a group assigned in a task, they will be able to claim the task and work on it. There is no trouble with that. The only thing that you cannot do is to create an fixed assignment in your process to an unexisting user. Let's say you create the Activity1 and you assign it to "Paul", but Paul doesn't exist yet, the task assignment will fail because Paul doesn't exist.
In the other hand if you define an Activity1 assigned to a group, let's say "Group(Operator)" then you can create Paul and if Paul belongs to the Operator group, he will be able to claim the task and work on it without troubles.
Greetings, hope it helps!