If you want to allow other Yumpu users to work with your account you can use the "Collaborators" feature. You need the WEBKiosk plan to do this. The first user you add is free, the others will cost monthly.
To add a collaborator go to "Edit my profile".
Scroll down to "Share account".
Click on "Invite a collaborator" and enter the e-mail address of the Yumpu account of the user you want to add.
The invited person can then confirm the invitation by clicking the link in the email. If the invitation is confirmed, the person can see login to your account under "Share Account".