FormsAuthentication を有効のままにして、次のメンバーシップ プロバイダーを追加できます。
<connectionStrings>
<add name="ADConnectionString" connectionString="YOUR AD CONNECTION STRING" />
</connectionStrings>
その後:
<system.web>
<authentication mode="Forms">
<forms loginUrl="~/Account/LogOn" />
</authentication>
<membership defaultProvider="MY_ADMembershipProvider">
<providers>
<clear />
<add name="MY_ADMembershipProvider" type="System.Web.Security.ActiveDirectoryMembershipProvider" connectionStringName="ADConnectionString" attributeMapUsername="sAMAccountName" />
</providers>
</membership>
</system.web>