Hi elkadiki,
I looked the link you provided and it's for radius not ldap, not sure if radius setup would be similar with ldap. I found another thread trying put the users into a group but he ran into problems and there was no solution provided in the end unfortunately (http://forums.juniper.net/t5/SRX-Services-Gateway/SRX-Dynamic-VPN-Active-Directory-Integration/td-p/245684).