I'm brand new and very confused with way PvP works on Warmane. How do I play against the best and largest population? Do I opt into Battlegroup 1with Icecrown and Lordaeron? If I do this, what are the consequences?
Is Seasonal mode a way to rank in the ladders or can you still do that with Battlegroup 1? Lastly, Does Blackrock seasonal have any benefits/are there any players from Icecrown and Lordaeron that compete here?
I don't know about population sizes. It's possible that battlegroup 1 is the biggest battlegroup. If you opt in you play against them, which includes lots of bis players, and you can't go back unless you create another character.
battlegroup 1 = Blackrock, Lordaeron, Icecrown
battlegroup 2 = Frostmourne and blackrock characters with "seasonal mode"
Potentially in battleground you may find players from every realms, regardless of battlegroup
For Arena you can only meet players from your same battlegroup instead
The most active is the "battlegroup 1" as you can easily realize by looking at population numbers at warmane homepage (or by looking at armory ladder)
Are you on blackrock? then I would recommend to leave "seasonal mode" by talking to that npc and selecting "battlegroup 1" mode. (once you do that visit again gear seller to get better free gear)