Organize group conference

Hello, tell me how to implement the following functionality:
Caller calls one group number, group call is initiated (called multiple handsets included in the group, in my case number 500), and then everyone who picked it up, gets in the conference, group call is not terminated, and it begin call other subscribers of the group(nex who pick up the phone also get to the conference room, and so on, while all subscribers will not enter the conference).
I realized Conference (room 900 and 990) and group call (call initiation caller groups - who first picked up the phone, and he says with the initiator call number 500)

Contents of extensions.conf


[Default]
exten => 101,1, Dial (SIP/101)
exten => 102,1, Dial (SIP/102)
exten => 103,1, Dial (SIP/103)
exten => 104,1, Dial (SIP/104)
exten => 105,1, Dial (SIP/105)
exten => 106,1, Dial (SIP/106)

; Group number, who first picked up the phone, and he says with the initiator call
; Group Call
exten => 500,1, Dial (SIP/101 & SIP/102 & SIP/103 & SIP/104 & SIP/105 & SIP/106)

; Conference
exten => 990,1, Answer ()
exten => 990, n, MeetMe ($ {EXTEN})
exten => 990, n, Hangup ()

exten => 900,1, Answer ()
exten => 900, n, MeetMe ($ {EXTEN})
exten => 900, n, Hangup ()

Contents of sip.conf

[101]
type = friend
host = dynamic
username = 101
secret = 123456
context = default
disallow = all
allow = g722

[102]
type = friend
host = dynamic
username = 102
secret = 123456
context = default
disallow = all
allow = g722

[103]
type = friend
host = dynamic
username = 103
secret = 123456
context = default
disallow = all

[104]
type = friend
host = dynamic
username = 104
secret = 123456
context = default
disallow = all

allow = g722

[105]
type = friend
host = dynamic
username = 105
secret = 123456
context = default
disallow = all

[106]
type = friend
host = dynamic
username = 106
secret = 123456
context = default
disallow = all
;


File contents meetme.conf
[General]
audiobuffers = 32
[Rooms]
conf => 990
conf => 900


Asterisk Version 11