How to bind two table in gridview in asp.net

I think you are looking at it from wrong angle. Look at it like this.

You populate the drop-down list with the values from the second table, rold Id and role names. Basically data-bind the whole table. Role Id goes as "value" member and role name goes as "Display" member.

The user selects one of the values from the drop-down list. Then while saving, you take user name, (and other details) and role id (value member) from the drop-down list and save it in db.