Here are the steps to assign a database role (eg: db_owner) to Multiple Logins:
1. Open SQL Server Management Studio (2005/2008). In the Object Explorer, after connecting to SQL Server, expand the Databases folder > Expand your Database Node (For demonstration purposes, I have chosen ‘Adventure Works’ database)
2. Now navigate to the Security > Roles > Database Roles folder. Double click on db_owner
3. In the Database Role Properties dialog box, click on the ‘Add’ button to open up the ‘Select Database User or Role’ dialog box and the enter the name of the user/role to add. You can even click on the Browse button to search the names.
Click OK and then OK again once you are done adding the users/roles to the Database login.