[GroupAPhones]
exten => 666,1,Dial(SIP/john,10,tr)
exten => 675,1,Dial(SIP/jane,10,tr)
....all of A-type users

[GroupBPhones]
exten => 688,1,Dial(SIP/bob,10,tr)
....all of B-type users

[GroupA]

include => GroupAPhones
include => GroupBPhones
include => SIPTrunk

[GroupB]

include => GroupAPhones
include => GroupBPhones