I think you'll find this solution to be plausible, since you don't mention how active they are. Because there was some Monumental growth that alternate history just needs to be tied to.
The concept of alternate history takes hold in the Proto internet of the lady teen hundreds when so many people were sending telegrams back and forth. Charles Dickens happens to pen a short story about luddites and trying to prevent the Industrial Revolution through an alternate history of what could have happened. Sherlock Holmes has one of his Mysteries then be about a time traveling Moriarty. Well these don't do great, but they do find a niche audience and people in Britain are chattering about it on that Telegraph system.
Alternate history is used in the first half of the 20th century by the British to basically talk about different kings and queens of Britain and sometimes of other European countries. It is an interest which catches the eye of some of the people at Bletchley park. When the British decide not to allow their computer work to become public knowledge due to security concerns, these people migrate to the United States.
Discussion of alternate history takes place between American and British computer enthusiasts throughout the 1970s and 1980s. Alternatehistory.com is said to be set up on the first FreeNets in the late 1980s, though it consists of the Usenet news groups at this point. Because the concept has gained popularity, there are many people who are interested in it and many others who subscribe to different alternate history fanzines.
When AOL starts sending their trial discs out in the early 1990s, someone in the business seized that alternate history is a big interest of people who also enjoy computers and decides that they will make a part of the AOL system the burgeoning alternate history group. As the number of AOL subscribers grows throughout the 1990s, a good number of those subscribers wind up getting into the alternate history site through AOL in their households.
As AOL declines and the internet takes over, alternatehistory.com is one of the first websites founded. The names of those who joined because of the AOL phenomenon are added as members in name even if they do not technically participate in the website because it is easier to just add them in then to send a mass email to millions of people because it's not as easy to send all those emails at once in the late 1990s.
By 2010 it should be fairly easy to have 10 million members, though most of them do not participate or even remember they were added.
The server would probably be getting kind of jammed at that point, though, so on January 1st, 2011, Ian sends out an email (it's now easier to send a mass email to all members) asking that anyone who wishes to continue participating in the program login and otherwise they will be dumped. The mass dumping of several million inactive email addresses occurs during the month of January in 2011.
Edit: wow, I just realized how old some of the people would be who don't even have memory of the mass AOL trial discs! I'm probably sharing history that some current members don't even realize happened!