6.5.How to create user logins to an already existing user?
In System ⇒ Security ⇒ Users, under section User logins click on + new button to add new user login to the selected user.
In the Add new login dialog, select a provider configuration that you would like to use when logging into the system. The providers, that where configured on the Login providers page (for more information check “Login providers”) can be selected here. Windows and Forms type login providers are enabled by default. If the user already has Forms and Windows user logins (like in this case), then these providers will not appear in the Login type dropdown list (multiple user logins are not enabled by these login types).