Ali Haider,
The radius attribute you are looking for is the Aruba-AP-Group attribute. The Value is attribute Number 10 and Aruba is Vendor ID 14823. Aruba sends the Ap-Group as a string value in authentication requests to the radius server. If you can configure your radius server to identify that value, you might be able to accomplish what you want:
(Aruba7640-US) #show aaa radius-attributes | include Aruba
Aruba-Auth-SurvMethod 39 Integer Aruba 14823
Aruba-Mdps-Max-Devices 18 Integer Aruba 14823
Aruba-CPPM-Role 23 String Aruba 14823
Aruba-Mdps-Device-Version 21 String Aruba 14823
Aruba-AirGroup-Shared-User 25 String Aruba 14823
Aruba-Device-Type 12 String Aruba 14823
Aruba-Mdps-Device-Imei 16 String Aruba 14823
Aruba-AP-Group 10 String Aruba 14823
Aruba-Named-User-Vlan 9 String Aruba 14823
Aruba-Mdps-Device-Serial 22 String Aruba 14823
Aruba-Mdps-Device-Name 19 String Aruba 14823
Aruba-AS-User-Name 29 String Aruba 14823
Aruba-AS-Credential-Hash 30 String Aruba 14823
Aruba-Auth-Survivability 28 String Aruba 14823
Aruba-Framed-IPv6-Address 11 String Aruba 14823
Aruba-Priv-Admin-User 3 Integer Aruba 14823
Aruba-AirGroup-Version 38 Integer Aruba 14823
Aruba-WorkSpace-App-Name 31 String Aruba 14823
Aruba-Template-User 8 String Aruba 14823
Aruba-Location-Id 6 String Aruba 14823
Aruba-Mdps-Device-Profile 33 String Aruba 14823
Aruba-AirGroup-Shared-Group 35 String Aruba 14823
Aruba-AirGroup-User-Name 24 String Aruba 14823
Aruba-Network-SSO-Token 37 String Aruba 14823
Aruba-AirGroup-Shared-Role 26 String Aruba 14823
Aruba-Mdps-Device-Iccid 17 String Aruba 14823
Aruba-Port-Id 7 String Aruba 14823
Aruba-No-DHCP-Fingerprint 14 Integer Aruba 14823
Aruba-Mdps-Provisioning-Settings 32 String Aruba 14823
Aruba-Mdps-Device-Product 20 String Aruba 14823
Aruba-User-Vlan 2 Integer Aruba 14823
Aruba-AirGroup-Device-Type 27 Integer Aruba 14823
Aruba-User-Group 36 String Aruba 14823
Aruba-AP-IP-Address 34 IP Addr Aruba 14823
Aruba-Mdps-Device-Udid 15 String Aruba 14823
Aruba-Admin-Role 4 String Aruba 14823
Aruba-User-Role 1 String Aruba 14823
Aruba-Essid-Name 5 String Aruba 14823