Using Spark for finding similar users to a user?
PMC from Mahout here- we're in the middle of a site re-org at the moment, and things are... well they're a mess. 

Here's a link to something I think is more useful. A tutorial on Co-Occurance in Spark. 

http://mahout.apache.org/docs/latest/tutorials/cco-lastfm/

Re "A Spark Library" well, mahout IS the spark library. 

To use Mahout (Scala only, sorry if you're a Python-phile, however the syntax, especially for Mahout is very pleasant), you either need to download mahout and run ./mahout spark-shell from the bin/ directory. Or if you like GUIs notebooks and Apache Zeppelin, check out this tutorial for setting up Mahout+Spark on Zeppelin 

http://mahout.apache.org/docs/latest/tutorials/misc/mahout-in-zeppelin/

(If you are compiling a Jar, you just add Mahout as a dependency.)