To Invite a User to your account either click on the 'Accounts' tab from the menu or the 'Share' button from the project panel.
From here you can see and edit the account settings. Click on the 'Members' tab to view the account members.
From here you can see all the account members. At the bottom there is a form to invite a new member. Enter their email and select which role you would like them to have, then click "Send Invite".
This will send them an email invite for them to open and create their User and have access to this account. They will appear in the account members list where any user's role can be changed the the role dropdown, or they can be removed from this account with the remove button. An account must always have at least one owner, so the last owner will not be able to be removed with the remove button.