From the Users tab, simply click on Add a User.
The process is similar to adding a user from a specific account, the main difference being that you can assign the user to multiple accounts and with different roles on the spot from Account Details.
Assigning a user to multiple accounts
Firstly, select the role you'd like them to have, then select the accounts you want to assign them to.
You need to choose all the accounts you want to assign this user to including subaccounts; in other words, if you assign a user to an account, they will not be automatically assigned to any subaccounts that parent account may have.
TIP: if you want to see a table with ALL accounts (in case you want to assign this user to all) click on View all Accounts and a table will display.
Adding a user with different roles to multiple accounts
Say you want a user to be admin of some accounts, but have a basic role in others.
Simply select Assign with another role to more accounts and you'll be on your way!