“Guest” inherits the privileges of “public.” The Database Owner and the owners of database objects can use grant and revoke to make the privileges of “guest” either more or less restrictive than those of “public.” See Chapter 12, “Managing User Permissions,” for a description of the “public” privileges.
When you install Adaptive Server, master..sysusers contains a guest entry. The installation script for the pubs2 database also contains a guest entry for its sysusers table.