We generally recommend using our grouped transactions feature for this use case. Assuming you footed the bill and your roommates end up paying you back their portion, group together all of these transactions and you should be left with a transaction that represents how much you paid. You can then treat this as a high-level transaction and assign a category to it.
Yes, after a few months we finally found the answer. Sadly, Mike is on vacations right now so I'm afraid we are not able to provide the answer at this point.