N-Way Calling or Three Way Calling by asterisk

How this works

When You speak with other party, press *0 (macro nway-start is executed). Called party is immediately transferred to free conference, and You get dialtone to enter number of party You want to invite. After call established and You talk to third user, You can press ** to invite him to conference and *# to hangup call and return to conference. From conference any user can invite anyone else by pressing 0 (all other steps are same as for *0)

Here is dialplan example:
[default]
exten => _XXX,1,Set(DYNAMIC_FEATURES=nway-start)
exten => _XXX,n,Dial(SIP/${EXTEN})

[dynamic-nway]
exten => _XXX,1,Answer
exten => _XXX,n,Set(CONFNO=${EXTEN})
exten => _XXX,n,Set(MEETME_EXIT_CONTEXT=dynamic-nway-invite)
exten => _XXX,n,Set(DYNAMIC_FEATURES=)
exten => _XXX,n,MeetMe(${CONFNO},pdMX)
exten => _XXX,n,Hangup

[dynamic-nway-invite]
exten => 0,1,Read(DEST,dial,,i)
exten => 0,n,Set(DYNAMIC_FEATURES=nway-inv#nway-noinv)
exten => 0,n,Dial(Local/${DEST}@dynamic-nway-dest,,g)
exten => 0,n,Set(DYNAMIC_FEATURES=)
exten => 0,n,Goto(dynamic-nway,${CONFNO},1)
exten => i,1,Goto(dynamic-nway,${CONFNO},1)

[dynamic-nway-dest]
exten => _XXX,1,Dial(SIP/${EXTEN})

[macro-nway-start]
exten => s,1,Set(CONFNO=${FindFreeConf()})
exten => s,n,ChannelRedirect(${BRIDGEPEER},dynamic-nway,${CONFNO},1)
exten => s,n,Read(DEST,dial,,i)
exten => s,n,Set(DYNAMIC_FEATURES=nway-inv#nway-noinv)
exten => s,n,Dial(Local/${DEST}@dynamic-nway-dest,,g)
exten => s,n,Set(DYNAMIC_FEATURES=)
exten => s,n,Goto(dynamic-nway,${CONFNO},1)

[macro-nway-ok]
exten => s,1,ChannelRedirect(${BRIDGEPEER},dynamic-nway,${CONFNO},1)

[macro-nway-notok]
exten => s,1,SoftHangup(${BRIDGEPEER})

Note, You need to provide FindFreeConf() function (or any other way) to get free Conference number.

And, to make all this work, here is features.conf:

[applicationmap]
nway-start => *0,caller,Macro,nway-start
nway-inv => **,caller,Macro,nway-ok
nway-noinv => *#,caller,Macro,nway-notok