The owners-autoassign plugin depends on the shared library owners-api.jar which needs to be installed into the $GERRIT_SITE/lib. You will then need to add the following entry to gerrit.config:
[gerrit] installModule = com.googlesource.gerrit.owners.api.OwnersApiModule
A restart of the Gerrit service will be required for the lib to be loaded.
Once the owners-api.jar is loaded at Gerrit startup, the owners-autoassign.jar file can be installed like a regular Gerrit plugin, by simply placing the file in the GERRIT_SITE/plugins directory or installed through the plugin manager.