sql web-application Connection String for member of a domain group
You need to configure the IIS Application Pool to use the identity (domain user) you created.

My example below shows the default, out-of-the-box configuration.  You'd need to choose 'Configurable' and specify the username and password there.

Then you need to configure the site to use the AppPool.



The connection string would then be:

server=myserver\sqlexpress;database=mydatabase;Persist Security Info=True;
Integrated Security=SSPI;Trusted_Connection=True; Pooling=True


Just like you have it.