From Jeff Hickman:

A guild earns experience and levels according to the accomplishments of its members. This isn’t a case of going out and hunting monsters or other players, and having “the score” translate as the guild’s score. “Leveling a guild is milestone based, and slow,” he told us, and in fact, his little chart indicated that the design calls for a guild to level from Rank 1 to Rank 100 in no fewer than eighteen months.

Rewards for leveling your guild will include (but not limited to) special group tactics, appearances (varying depending on the sort of guild – an intriguing statement, but not elaborated upon), and special abilities. Potions, emblems, titles, and craft resources are all on the table.

By choosing member milestones instead of raw numerical totals as the basis of Living Guild advancement, smaller guilds have a solid chance of earning the rewards. (And though it is not certain at this time, the designers are leaning towards guilds retaining their rewards even when individual contributors leave the group.)