From this SOE post on the Test forum:
Currently when someone activates Fellowship Experience Sharing, a portion of the experience that they earned is shared with other members of their Fellowship who also have Fellowship Experience Sharing activated. This is directly applied to other members of their Fellowship who also have Fellowship Experience Sharing enabled.Click on the SOE forum post link to read more and comment on the proposed changes.
After the next patch to the Test Server, this will substantially change. Going forward, instead of experience being directly applied to Fellowship members, this experience shared will go into a bank (specific to each member) called Vitality. So If Player A is earning 1000 experience per kill, and is sharing with Player B, C, and D, then players B, C, and D will each be receiving 250 exp per kill into their individual Vitality banks.
Once a player with Vitality experience logs in, they will receive bonus experience for any kills they are credited for, allowing them to gain experience at a vastly accelerated rate*. This bonus experience is paid out of the Vitality experience they have accumulated, and once they run out out vitality they will no longer receive bonus experience. This bonus experience will be applied at a rate of up to 5x if doing normal experience, or up to 3x if doing AA experience.
In addition, when a member of a Fellowship has reached the maximum limit of Vitality experience, Fellowship Experience Sharing will be automatically toggled off for them. The current limit on Vitality experience is enough regular experience to gain 10 levels or enough AA experience to take the player to their AA cap. Vitality experience does not expire, and is permanent (until used). If a player cannot gain normal experience, they cannot store Vitality experience.
P.S. In addition, it is expected that the game will soon be changed so you may only join a fellowship once every 20 hours.