Starting with Newcastle, naturally, there are 8 Ukestras (ukulele orchestras) and two choirs run by the monumentally committed, energetic and fun people at The Sum of the Parts (music).
Below are links to other groups in Australia that provide people with places to meet, maybe learn a bit and play ukulele music together! Let me know about your group and I'll add a link or contact details to the list.
for groups in the USA and Canada, this list at Ukulele Hunt is very comprehensive
Send me a link if you'd like your group included.