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.