Field order in a composite index order with high selectivity and low selectivity fields
As an alternative to @AaronBertrand's solution (if you can't or don't want to create an indexed view), I would recommend you to create an index on (Enroll_Date, UserID). If this type of question is very common on your table, this should probably even be your clustered index.

I would not generally recommend high-selectivity indexes as a general "best practice", but rather look at what index will give your query the best performance.

An index on (Enroll_Date, UserID) will give your query a highly optimized, non-blocking query plan with Stream Aggregates.



"Non-blocking" in this context means that the query doesn't need to buffer any significant amounts of data (like, for instance, a sort or hash aggregate would), which means it (a) starts returning rows immediately, and (b) consumes practically no working memory.