It would take a dev to give an accurate response to that, but my personal opinion is that US3 is the most active at this point in time. I believe they are all active enough to support group play.
I don't know if you have made a character yet but the first one will be created on whichever server the devs have set as the default for your region. If you want to play on another server than that, you can choose which server to create your characters on after that point. You can have up to three free characters per server and can purchase an additional two character slots per server with Crowns. You can not transfer characters and items between servers but. Crowns and Ardent Society membership are account based and can be used to the benefit of all characters on all servers.
Welcome! I am new too! I really like this game though. Love the fact that I can hop in the game on my phone at lunch or slow times at work and gather, craft, quest or whatever then hop on the PC at home for extended play sessions. Now if they would just port it to Xbox so I could kick back on the couch and play on the big screen?
Do note, you shouldn't pick server just because it's more populated. Each server has different age demographics as well as a differing maturity of player level. Usually us3 also has more lag spikes, more issues that require mods, etc.