When trying to convert a standalone mongod to a replicaset, or add members to an existing set in a local environment, you need to specify the dbpath in order to avoid getting an assertion error when bringing up multiple instances on multiples ports.

info on setting --dbpath in the commands should be added to the following pages:

http://docs.mongodb.org/manual/tutorial/convert-standalone-to-replica-set/

http://docs.mongodb.org/manual/tutorial/expand-replica-set/