Google allows multiple users to access a single Gmail account, this is also known as email delegation. It should be noted that when this feature is enabled, all new messages are marked as read if they are opened by the other users. However, it is possible to leave conversations unread.
Sign in to your Gmail account and click on Gear button. Then, select Settings:
Go to the Account and Import section. Next to “Grant access to your account,” select Leave conversation unread when opened by others: