Create a web service that retrieves the users and groups, and then populate
the drop-down list box using the webservice as a data source.
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S.Y.M. Wong-A-Ton

If your LDAP directory is on windows, you could use ADSI to write a
script in the "On Load" event of your infopath form to load the users
into the dropdown list.

http://www.codeproject.com/asp/Exchange_ADSI_LDAP.asp
http://www.codeproject.com/asp/serach_exchange_asp.asp

If the LDAP directory is not on windows, you'll need to create a web
service interfacing with it, ala

http://www.codeproject.com/vb/net/LDAP_Using_VBnet.asp