Under "Settings" -> "User management" you will find a list of all the users who have access to the account.
You can easily add and remove users from this page. You need "owner" rights to be able to remove users from an org.
To invite new users, input the email address of the person you want to invite, and then click "Send invite". Subsequently, the invited user will receive an email containing instructions on how to proceed.
Please note: The invitation remains valid for 24 hours only, and can be revoked if necessary.
If you are managing multiple organizations, double-check which one you are currently managing before sending invites.
Switch organizations by using the drop-down menu in the upper-left corner.