There are two different things that can be done. The first one is letter B. A flow will be created on the user object so that a new permission set will be assigned. The second thing to do is letter C.
A lightning process can be created so that a flow can be launched with ease. The specific permission set if done correctly will only be available to new users.
For example, there may be some things that they cannot access yet because they are new. They will eventually be part of another group the more that they prove that they are not just new. At times, there may be some tools that will only be available to those who are new to the website.