Members of the LEGO Message Boards or "users" as they're usually often called, are users of the MB's who post to topics in it. To be a official user of the MB's, a person must sign up for an account to join in and post. Most users chose to post in the MB's while some of them are Reader's who don't post but give likes to post instead. Other users are Official's, who are the Moderator's of the MB's, Staff, who are special guests to the MB's, and also LEGO Education Official's who work in the educational Forums.
There are also users from before the update of 200# who still show up from time to time in the MB's more commonly in LMB Old Timers! topic.