When converting a replica set with an arbiter to a shard, does the arbiter also get added to the shard?
When you add a new shard using sh.addShard(...) you are providing a seed list of members which will be used to discover the current replica set configuration. You can specify as few as one member of the replica set or as many as all, and you do not need to specifically include the arbiter.

FYI, the sh.addshard() mongoshell helper wraps the underlying addShard command (which has a few additional options not currently exposed via the sh.addShard() helper).