use the server validation included with CSS Form builder to validate the registration page.
Go to insert -> WebAssist -> CSS Form Builder -> Server Validation. Select the required Not Blank validation, then click the lightning bolt next to server validation and select the User name form element, next click the plus button to add it to the list of validations.
repeat the process for each form element that should be required.
to show an error when validation fails, type the error in design view, the on the server behaviors panel, click the plus button and select Validation Toolkit -> Validation Show If. from the Validation fail list select the appropriate validation to trigger the error to show.
when a user logs in, the ID is stored in a session variable, you can use this session variable to filter your recordsets.
Refer to the recordset created on the profile page to how it is filtered, it will use the same session variable.