Hello mrobben375515 to do this first go to your database table and add the unique option to the cell where the user name is then go to your registration page or where you add users and add a new recordset of table that has the users now filter it but form variable for the form that contains the login or unique name example if your form has login password adress name etc... you would filter the recordest with form variable login.
then in security assist add a new rule on the server side validation, choose validation type: number and server variable add your recordset total example $totalRows_userRecordset, this will ask you what kind of number select (internactionl 1000,00) on minimum place -1 and maximum 0 and on allow decimals 0, now go to the form field where users place their login and add next to it <span> Login name already in use </span> then select your span and go to the server behavoirs of security assist and select show if and add it there :)
Hope this helps
Sades
