Page groups are found in the extensions_additional.conf file in Asterisk. The paging group portion of this file should read as follows in which "101 and 102" are the usernames and extensions of two speakers and "103" is the extension used to call the paging group of speakers 101 and 102, and "105" is the extension to call speakers 101,102, and 104:

[ext-paging]
include => ext-paging-custom
exten => PAGE102,1,GotoIf($[ ${CALLERID(number)} = 102 ]?skipself)
exten => PAGE102,n,Dial(SIP/102,5)
exten => PAGE102,n(skipself),Noop(Not paging originator)
exten => PAGE101,1,GotoIf($[ ${CALLERID(number)} = 101 ]?skipself)
exten => PAGE101,n,Dial(SIP/101,5)
exten => PAGE101,n(skipself),Noop(Not paging originator)
exten => PAGE104,1,GotoIf($[ ${CALLERID(number)} = 104 ]?skipself)
exten => PAGE104,n,Dial(SIP/104,5)
exten => PAGE104,n(skipself),Noop(Not paging originator)
exten => Debug,1,Noop(dialstr is LOCAL/PAGE102@ext-paging&LOCAL/PAGE101@ext-paging)
exten => Debug,n,Noop(dialstr is LOCAL/PAGE104@ext-paging&LOCAL/PAGE102@ext-paging&LOCAL/PAGE101@ext-paging)
exten => 103,1,Page(LOCAL/PAGE102@ext-paging&LOCAL/PAGE101@ext-paging)
exten => 105,1,Page(LOCAL/PAGE104@ext-paging&LOCAL/PAGE102@ext-paging&LOCAL/PAGE101@ext-paging)

; end of [ext-paging]