Account > Members > Choose Account Level/Project Level/Pending
Click the Account icon in the gray navigation bar. Click Members. The page will provide three tabs: Account Level, Project Level, Invited/Pending.
To add someone to the account, go to the blue +Add Account Member button. A pop-up window will appear titled "Invite New Account Member". There is a blue information button (blue circle with an "i"). Click this button and the role definitions will appear. The same can be done when ready to add Project members.