Please search before asking, this has been asked hundreds of times.
Short answers, there is no way todo this with CUCM.
Your options are either use CUC/Unity to front end the meetme and announce who's trying to join (if you're interested, search CSC for detailed instructions)
Or if you really want a PIN you'll need to do something with UCCX/UCCE or TCL scripting.
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HTH
java
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