Instant Adventure is fairly simple: join into a group just as you do when joining a dungeon and you'll be given a random outdoor quest in a level-appropriate area. The quests scale to the size and level of the group, rolling into new quests as the old ones are finished. Eventually it'll peak, hitting a rare monster and giving everyone involved sweet loot, before dropping back down and continuing to push the mass of players around the level. It sounds like a great addition for both hardcore and casual players alike, as it'll give those who have done everything infinite new content to do, and those who don't have time to do everything a chance to participate in big groups for as little (or as long) as they wish.
We'll have more about Rift next week, so check back if you're interested in finding out even more awesome Rift news.