Users are representations of people in the solutions. As such, they have roles, are on teams, have attributes they can change (eg their name), and attributes they cannot (eg their manager).
There are multiple ways to go about creating an individual user:
Each of these has it's own advantages. Import may be practical if the user already exists in another platform space or environment. Inviting is quick and simple. Create offers you all the user configuration options with a clean slate, and Clone offers you, essentially, a preconfigured user.
However you create a user, they will get an invitation to the system and have the ability to set their own password upon their first system entry.
Once a user is created, you can update the user by clicking on their name in user user list at Home -> Settings -> Users.
From within the user, adding and removing the user from teams is as simple as checking and unchecking the boxes for the team: