[OpenSER-Users] Restricting calls between users

Regards, I have a debian (etch - kernel 2.6.18) server with openser 1.1.0-9-etch1installed and Asterisk 1.4.6. To admin openser, I'm using openseradmin 0.3 and Asterisk-GUI (Rev. 1203) to admin Asterisk. Openser service works fine to make calls between users (Linksys phones, ATA phones, Sipura phones, etc.) and I installed Asterisk to use VoiceMail, Call Queues, Conferencing, Voice Menus, Calls redirections and PSTN GW functions. NOTE: I can hear only Asterisk VoiceMail, but I'm still working to openser-asterisk integration. For me, who I am a new Linux user, It's hard but not impossible. I want to configure call restrictions between openser VoIP groups at the same domain. For example: I have an IP Network with clients who has remote offices. I'm using openser to connect their offices. Now, I want to use groups to resctric calls between clients. I want to know what's the best solution to do calls restrictions between groups. I must use AVPops, Groups, Asterisk modules, etc ?????? Can anybody help me? PD: I'm sorry for my spanglish. Thanks