Daniel Calbimonte adds a new user two separate ways:
Open PGAdmin and connect to the server where you want to create the new user.
In the Object Browser, expand the Server and go to Login/Group Roles tree, and right-click on the folder. Select Create>Login/Group Role option from the context menu.
In addition to adding a user via PGAdmin, there’s also a script to add users via pgSQL, and that will look a lot more familiar to SQL Server administrators.