If I remember wel I picked this out of the Asterisk Cookbook. This is a copy of a working example. If you press 0 while in the conference you will be asked to enter the number of the person you want to join the conference. Not sure if this is what you were looking for but perhaps it is helpful.
/etc/asterisk/confbridge.conf ; I copied it from the bottom of the file
[volume_ctrl_menu]
type=menu
*5=toggle_mute
1=increase_listening_volume
4=decrease_listening_volume
7=reset_listening_volume
3=increase_talking_volume
6=decrease_talking_volume
9=reset_talking_volume
0=dialplan_exec(conference,conference-joiner,1)
/etc/asterisk/extensions.d/conference_with_features.conf ; this is a new file
[conference]
exten => conference-entry,1,Answer()
exten => conference-entry,n,ConfBridge(9000,volume_ctrl_menu)
exten => conference-entry,n,Hangup()
exten => conference-joiner,1,NoOp()
exten => conference-joiner, n,Read(numberToDial,vm-enter-num-to-call)
exten => conference-joiner, n,Originate(SIP/{numberToDial},exten,conference,conference-entry,1)
exten => h,1, NoOp(“dialplan is in h extension”)
exten => h,n, GotoIf($[ “${CONFBRIDGE_RESULT}” : “HANGUP”]?ok)
exten => h,n, NoOp("value of CONFBRIDGE_RESULT is not HANGUP) ; or whatever you want to happen
exten => h,n, NoOp(the value is " ${CONFBRIDGE_RESULT})
exten => h,n, Goto(h,end)
exten => h,n(ok), NoOp("value of CONFBRIDGE_RESULT is HANGUP) ; or whatever you want to happen
exten => h,n(end),NoOp(“this is the end”)
/etc/asterisk/extensions.conf
#include extensions.d/conference_with_features.conf ; at top of the file