My nieces first baby is less than a month old, this is the first grandchild among my siblings. Ergo, everything in this babys life is, essentially, the first time also for us. My brothers first Rosh Hashana as a grandfather, mine as a great aunt, etc. And were scattered across the US so well all be at different shuls. My question, then, is when is Shehecheyanu said, and who all says it? I am wondering specifically for this Rosh Hashana.

Mazal Tov! Shehechiyanu is recited on any really good news that makes one really happy. Similarly, regarding birth, any relative who is really happy can say the bracha (although usually it's said by mainly by parents, grandparents and siblings), and it should be said as soon as possible upon hearing the good news, regardless of Rosh HaShana.