Hi ya'll, this is my first post here. I've been using a find/perl line for years to accomplish what you want. Here it is:

Code:
find /some/starting/dir -type f -name '.htaccess' -depth \
-exec perl -pi.BAK \
-e 's#Require user userid_a, userid_b, userid_c#Require user userid_a, userid_c#g' {} \;
For each .htacess file found perl will first create a .htaccess.BAK file for those "oh crap I made a big mistake" times. I hope it's helpful to you.