Hi Stuart,
Yes, you can add Readers and Writers for all document types. You saw this in the installer but you can modify this in web.config.
In web.config you'll also see the DBXLAdmin group:
<add key="BrowserGroupAlias" value="NT AUTHORITY\Authenticated Users" />
<add key="ReaderGroupAlias" value="INTRANET\DbxlUsers" />
<add key="WriterGroupAlias" value="INTRANET\DbxlUsers" />
<add key="AdminGroupAlias" value="INTRANET\DbxlAdmins" />
<add key="DbxlAdminAlias" value="INTRANET\DbxlAdmins" />
In the example above we have only two groups: DbxlUsers (who can read and write) and DbxlAdmins (who can manage DBXL and manage every doctype).
If you need to make changes, modify the web.config. Let us know if you run into any issues.