From Online Manual
Members have access to Profile and Notification Options that are unavailable to guests. Members can also be placed in Membergroups, made [Moderators]] or Administrators, given special Permissions, and access members-only areas. Being able to send Personal Messages, receive forum e-mails, and post new Polls and Topics are some additional benefits that may be available by becoming a member of a Forum.
By default, a Forum can support 16,777,215 registered members. However, this limit can be raised relatively easily.