User created using 'Add Person' can't access DBs over the web through groups

We have a web application and some of our users are not registered users. If we add the non-registered users to the ACL using their username, they can access the database over the web but if we put these users into a group and add the group to the database, they get the message "You are not authorised… ".

Is this because groups only work with registered users? I can’t find any documentation to back this up but it seems to be the logical answer - does anybody know?

Thanks!

Pip