Is the beta invites limited to 1 device or can we join multiple ?
I will need to get a new sd card before I can revive my pi3, I have a few of the first versions, but if its limited to one device I dont want to burn it on those 
By the way, is there any particular reason for the “Version 2 or 3 recommended” on the getting started page ?
The Raspberry Pi 2 and 3 both have quad-core processors and 1 GB of RAM, where the the Pi Zero and older Pi's have only one core and 512 MB of RAM. Also, the Pi 2 and 3 both have onboard ethernet, and the Pi Zero does not. In addition, the Pi 3 has onboard wifi.
You can still run particle firmware on the Pi Zero, but due to it having limited network connectability (you must use an adaptor to get Wi-Fi or ethernet) and reduced processing power it is more difficult.
You can always transfer the SD card between Pi's or just copy the entire content to another SD card. I don't think the beta limits the number of Pi's you can add, it just limits the number of users.
That's exactly right. You should currently be able to register up to 25 Pi's to a single Particle account for no cost.
Very nice 