The Guest user is an account which is automatically created when a database is created. This account is used when a login connects to a database without having a user account created in it. By default, guest is disabled, to enable and disable, use the codes below;
The recommended way of managing guest account is, have it in disabled status. If you have enabled it for some reason, permission can be granted either using graphical interface or through T-SQL.
If guest is enabled in AdventureWorks2008 database and above permission is granted, when a login (eg. Jane) connects to AdventureWorks2008, she will be able to read data from Sales.Customer though she has no user account created in AdventureWorks2008.