Confirmed regions
The Global Launch FAQ names North America, South America, Europe and Japan. Choose based on latency, schedule and the people you plan to play with.
Cross-launcher play
Steam and PURPLE are not separate server ecosystems. Players using either launcher can join the same Global service.
Friends and full servers
Coordinate the region and server before creating characters. Official announcements say transfers will be available in the future, but exact rules should not be assumed until published.