The options d or D dynamically open conferences; conferences are defined statically in meetme.conf.

Internal help for this application in Asterisk 1.4: 

  -= Info about application 'MeetMe' =- 

[Synopsis]
MeetMe conference bridge

[Description]
  MeetMe([confno][,[options][,pin]]): Enters the user into a specified M
eetMe
conference.  If the conference number is omitted, the user will be promp
ted
to enter one.  User can exit the conference by hangup, or if the 'p' opt
ion
is specified, by pressing '#'.
Please note: The Zaptel kernel modules and at least one hardware driver 
(or ztdummy)
             must be present for conferencing to operate properly. In ad
dition, the chan_zap
             channel driver must be loaded for the 'i' and 'r' options t
o operate at all.

The option string may contain zero or more of the following characters:
      'a' -- set admin mode
      'A' -- set marked mode
      'b' -- run AGI script specified in ${MEETME_AGI_BACKGROUND}
             Default: conf-background.agi  (Note: This does not work wit
h
             non-Zap channels in the same conference)
      'c' -- announce user(s) count on joining a conference
      'd' -- dynamically add conference
      'D' -- dynamically add conference, prompting for a PIN
      'e' -- select an empty conference
      'E' -- select an empty pinless conference
      'i' -- announce user join/leave with review
      'I' -- announce user join/leave without review
      'l' -- set listen only mode (Listen only, no talking)
      'm' -- set initially muted
      'M' -- enable music on hold when the conference has a single calle
r
      'o' -- set talker optimization - treats talkers who aren't speakin
g as
             being muted, meaning (a) No encode is done on transmission 
and
             (b) Received audio that is not registered as talking is omi
tted
             causing no buildup in background noise
      'p' -- allow user to exit the conference by pressing '#'
      'P' -- always prompt for the pin even if it is specified
      'q' -- quiet mode (don't play enter/leave sounds)
      'r' -- Record conference (records as ${MEETME_RECORDINGFILE}
             using format ${MEETME_RECORDINGFORMAT}). Default filename i
s
             meetme-conf-rec-${CONFNO}-${UNIQUEID} and the default forma
t is
             wav.
      's' -- Present menu (user or admin) when '*' is received ('send' t
o menu)
      't' -- set talk only mode. (Talk only, no listening)
      'T' -- set talker detection (sent to manager interface and meetme 
list)
      'w[(<secs>)]'
          -- wait until the marked user enters the conference
      'x' -- close the conference when last marked user exits
      'X' -- allow user to exit the conference by entering a valid singl
e
             digit extension ${MEETME_EXIT_CONTEXT} or the current conte
xt
             if that variable is not defined.
      '1' -- do not play message when first person enters

diff output to internal help in Asterisk 1.2: 

8,11c8,11
<   MeetMe([confno][,[options][,pin]]): Enters the user into a specified
 MeetMe
< conference.  If the conference number is omitted, the user will be pro
mpted
< to enter one.  User can exit the conference by hangup, or if the 'p' o
ption
< is specified, by pressing '#'.
---
>   MeetMe([confno][,[options][,pin]]): Enters the user into a specified
 MeetMe conference.
> If the conference number is omitted, the user will be prompted to ente
r
> one. 
> User can exit the conference by hangup, or if the 'p' option is specif
ied, by pressing '#'.
20,21c20,21
<              Default: conf-background.agi  (Note: This does not work w
ith
<              non-Zap channels in the same conference)
---
>              Default: conf-background.agi
>              (Note: This does not work with non-Zap channels in the sa
me conference)
27,30c27,28
<       'i' -- announce user join/leave with review
<       'I' -- announce user join/leave without review
<       'l' -- set listen only mode (Listen only, no talking)
<       'm' -- set initially muted
---
>       'i' -- announce user join/leave
>       'm' -- set monitor only mode (Listen only, no talking)
32,35d29
<       'o' -- set talker optimization - treats talkers who aren't speak
ing as
<              being muted, meaning (a) No encode is done on transmissio
n and
<              (b) Received audio that is not registered as talking is o
mitted
<              causing no buildup in background noise
41,42c35
<              meetme-conf-rec-${CONFNO}-${UNIQUEID} and the default for
mat is
<              wav.
---
>              meetme-conf-rec-${CONFNO}-${UNIQUEID} and the default for
mat is wav.
52d44
<       '1' -- do not play message when first person enters