My mistake. I would have the host send a message with the relevant peer's index. The peer can then store it in a variable locally to access if necessary. Either do it on request by having the peer send a message requesting the number and the host responds with it, or do it on peer joined have the host send the number and the peer save it.
Or if peers need to be constantly updated as people leave and join, the host can just broadcast the relevant information regularly via messages.