If you are using ASP.NET 2.0 membership, all your users will be stored under tha App_Data folder in ASPNETDB.MDF database. If you want to open the database, you can do so by connecting to your SQLEXPRESS server. You have to launch SQL server Management Studio and use the connect Object Explorer to connect to COMPUTERNAME\SQLEXPRESS where COMPUTERNAME is the name you’ve given to your computer.
Once you are connected, right click on ‘Databases’ and click on ‘Attach’. A window will pop up and you can click on the ‘Add’ button and browse to the location of the ASPNETDB.MDF file. You can find this location by clicking on the properties windows in Visual Studio 2005. Once this is done, click ‘OK’ at the bottom on the pop up window and you will be able to view the contents of the MDF file.