1) Click the profile avatar on the top right-hand side of the navigation bar.
2) Choose the 'Account Details' option from the dropdown menu (indicated by the red box below).
Under the first tab 'My Profile' you'll find the option to update your name, email and password.
The next tab is 'My Company' and this is where you can update any part of your company information such as address or phone number.
Finally, under the 'Users' tab, you can invite / add new users. There is no limit on how many users you can have, and there are no financial implications for adding additional users.
To find out more about users, read this useful FAQ.