How do you grep results from 'find'?

Use the -exec {} + option to pass the list of filenames that are found as arguments to grep:

find -name Gruntfile.js -exec grep -nw 'purifycss' {} +

This is the safest and most efficient approach, as it doesn't break when the path to the file isn't "well-behaved" (e.g. contains a space). Like an approach using xargs, it also minimises the number of calls to grep by passing multiple filenames at once.

I have removed the -e and -r switches, as I don't think that they're useful to you here.

An excerpt from man find:

-exec command {} +
This variant of the -exec action runs the specified command on the selected files, but the command line is built by appending each selected file name at the end; the total number of invocations of the command will be much less than the number of matched files.

While this doesn't strictly answer your question, provided you have globstar turned on (shopt -s globstar), you could filter the results in bash like this:

grep something **/Gruntfile.js

I was using religiously the approach used by Tom Fenech until I switched to zsh, which handles such things much better. Now all I do is:

grep text **/*(.)

which greps text through all regular files in current directory.

I believe this to be much cleaner syntax especially for day-to-day work in shell.