When creating a new user, you'll have the option to clone an existing user's permissions after setting up the user's login credentials and campaign access.
Afterwards, you'll see all of your other users listed. Choose one, and their permissions will be copied to the new user. Click continue once you've chosen the permissions you'd like to use.
Note that you can only clone permissions from the same type of user: When creating clients you can only clone the permissions from other clients, and when creating staff you can only clone the permissions from other staff users.