[R] generating clustered data

[Brian Perron]

Hello R folks,

Does anybody have code to share for generating (via simulation) 
clustered data?  The type of data I am looking to simulate would allow 
fitting of a multilevel model with random intercepts.   I looked at the 
mvtnorm package but am not quite sure how to create clusters.  (Can this 
be done by simply changing the seed?)  If somebody could point me where 
to look for the relevant code or perhaps send some sample code, that 
would be great.

Thanks,
Brian