SSRS group adding

Hi everybody, 

I have an SQL server with SSRS. The reports work fine, but the reports are only available with the administrator profile. So I thought I would like to add another "profile" which is only for reading data. 
I created a new AD group, which is 'DOMAIN\users_test' but when I want to add the group on SSRS webpage I've got the following error message: 
"The user or group name 'DOMAIN\users_test' is not recognized."
I have no idea what should I do. 

Can you give me some advice? 
Thank you.
PN