Welcome to Guru!
1) Slowly dying out, but still somewhat populated for much of the game. Don't expect finding random groups for non-storyline activities in-game, but that is where your friends list and Guild come in! PvP is very active, although the Codex Arena often has wait times in-between matches except during peak hours.
2) Guild Wars has the best PvP of any MMO around, hands down. There is everything from the casual Competitive Missions and Random Arenas to more hardcore Guild versus Guild and Heroes' Ascent.
3) Guild Wars is balanced by the rock-paper-scissors concept, meaning some roles are geared towards winning versus other roles. All classes are played extensively in PvE, although in high-end PvP the current meta leaves Dervishes out in general.
4) To get into a Guild, you must be invited by a Guild Officer or Leader. There are plenty of PvP Guilds around, but most will expect you to have a good handle on the game, seeing as it has been out for 5 years. You're best off spending a while in PvE before going to PvP and joining a PvP Guild.
5) Get the free trial at
www.guildwars.com and you'll get a very basic feel for the game. You won't get very far in PvE, and will probably get utterly dominated at PvP, but you'll get the gist.
Let me know via PM here if you get the trial and would like to chat in-game!