Grouping two sets of data
Your performance is going to suffer until you can create a "WHERE IN"-style query for users, either by building a string or by combining Expressions. 

Or, if you want to keep it simple, just put the active criteria in your query, don't pull it up front. I'd recommend this approach.

Add .Expand("Users") on the active subscriptions query to pull back the related Users. I would avoid getting your ActiveBuilderSubscriptionSummary involved until you get your results back, at which point you can then project the query results to a collection of ActiveBuilderSubscriptionSummary objects using a standard LINQ .Select.

If you are hesitant to remove or change your method because you need to reuse it, or it is comfortably nestled in a repository, I'd refactor to use a stored proc instead. Otherwise, faced with the choice, it is way better to duplicate your query in code than to force an eager load of data up front.