Account > Members > Project Level
Click the Actions drop-down menu in the far-right column, then click Delete to remove the member.
You may also click on edit in the Actions drop-down menu to edit the role of a member.
To add a new person to the Project:
Account > Members > Project Level > +Invite Project Member
Enter the email address of the new member, find the designated project in the drop-down, choose which role the new member will have, then click send invitation
To add a new person to the Account:
Account > Members > Account Level > +Invite Account Member
Enter the email address of the new member, click the drop-down arrow, and choose which role the new member will have, then click send invitation