What happens to my mining rewards when more people join the network?

Your share of the rewards goes down. The Malairte protocol adjusts difficulty so that blocks come out at roughly the same rate regardless of how much total hashrate is mining. When new miners arrive, the network gets harder and the same hardware earns a smaller slice of each block reward. This is by design and it is why honest profitability math has to account for difficulty going up over time, not just price. A coin getting more popular among miners is a headwind for any individual miner, even if it sounds like good news.