What if I co-wrote a song with someone else? Do we both need to become Members of the MLC? And who needs to register the song?

Who needs to become a Member? 
The answer depends on which parties have the right to register the co-written musical work (song) with The MLC and collect the mechanical royalties. If co-writers create a legal entity, which they co-own, and which has been assigned the right to register the co-written song with The MLC and collect the related mechanical royalties, then only the legal entity needs to become a Member. If each co-writer has retained their right to register the song with The MLC and collect their own mechanical royalties, then each co-writer needs to become a Member of The MLC.

Who needs to register the song? 
Each party who has the right to register their own songs with The MLC, and collect their own mechanical royalties, should register their share of the song. For example, if there are two self-administered songwriters, composers, or lyricists who co-wrote a song, and each one has the right to register their own share and collect mechanical royalties for their own share, then each party should become a Member of The MLC and separately register their share of the song. In contrast, if co-writers create a legal entity, which they co-own, and which has been assigned the right to register the co-written song with The MLC and collect the related mechanical royalties, then the legal entity will register 100% of the song and collect the royalties.