Sitecore uses the standard ASP.NET membership provider for its users, so you can create users with basic ASP.NET calls.
userName = string.Format(@"{0}\{1}", domain, userName);
Membership.CreateUser(userName, password, email);
domainthe variable will be sitecoreor extranet.
Karina Apostolides has a set of useful membership features available here: Programmatically add and edit users
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