Get the subdomain like this
string domain = Request.Url.Authority;
string subDomain = domain.Split(new char[] { '.' }, StringSplitOptions.RemoveEmptyEntries)[0];
You can then use that to get the correct connection string. Maybe call your connection string "user1" so you can get it direct from the ConnectionStrings collection. Google "asp.net get connectionsrting" if you don't know how to configure connection strings.