The multi-engine land rating is an “add-on” to an existing single-engine land private, commercial, or ATP certificate. For this rating, both VFR and IFR operations will be emphasized. There are no minimum time requirements for the multi-engine land rating; however, you will need an instructor endorsement for flight and ground training prior to taking the checkride. There is no written exam, only an oral exam and practical checkride are required.
Multi-engine rating can be added to a private pilot certificate, in most cases it makes more sense to wait until a pilot has a commercial pilot certificate and instrument rating as this will save the pilot a substantial amount of money in the long run.