Regularly occuring events like
tournaments and
leagues have no signup thread.
Tournaments are weekly and happen on every Saturday in a rotating timeslot. Each tournament has its own unique set of rules and restrictions. 3/4 of tournaments are unupgraded.
Here is this week's tournament.
PvP Leagues go in 3 month long seasons, with close to no restrictions (shards are banned by default, but this can be waived if both parties agree). Leagues work off of an elo system, and the top 6 at the end of each season win prizes. Season 1 of 2017 has just started.
Battle League does not allow upgraded cards, while
Championship League does. People normally find others in
chat to find matches.
Most other
pvp events have a signup thread that you post in before the event starts. The ongoing
Trinity event and the biggest,
War, are examples of this.