The promotion of guests is one of the room settings. 

Log into your scribblar account at scribblar.com/login and find the room you'd like to edit. Click edit.

Scroll down and make sure that the 'Promote Guests' option is selected. If it is not then select it.

Save your room settings by clicking 'Update'.

Any guests visitign this room from now on will beautomatically promoted to Moderator status and will have access to all the tools in the room. They wonl;t however be able to promote and demote other users, or kick users out of a room. Only users in the Admin role can do that.

For more information on roles please see this article.