I'll be playing at release barring some unforseen catastrophe to the game.
Minor rifts can be closed by 1 person trivially. Need 2 people to beat stage 5.
Major rifts require 2 people to close. Need about a group to beat stage 5 (reliably).
Zonewide events can be won with 6 people (5 defend, 1 attack), though it'd seriously test your patience. The lowest I've won them is with 20, though I think we started with only 10 people and got the boss halfway down before all the vultures jumped in for the free loot!
Now it's certainly possible to lose to these rifts if your skill/coordination sucks, but I never said it's easy.
In fact one of the best way to level up seems to be just not close any rifts when you're attacked by a zonewide event and just keep on kill the invasions that attempt to attack an ultimate wardstone. If you think about it, it's basically a 20% XP buff for defending an ultimate due to the aura.
For my leveling spec I did Reaver/Champion/Riftblade, Reaver mostly for AE aggro/lifetaps, Champion for charge (upgrade charge very useful in PvP), and Riftblade for the crit damage buff (works well with Inescapeable Fury talented). It's mostly a tank spec but it does enough damage to be useful. I've been messing around with Void Knight and it seems like it's the best defensive tank, even better than the Paladin. If you manage your pacts right you have 20% spell reduction, +50% armor, and +50% strength at all times. It's also one of the overpowered specs for PvP because people never figure out that you're not supposed to cast on Void Knights. That said, keeping the pact buffs up is pretty challenging and you can go from a hero to a zero in 5 seconds!