I think the answer to your question depends on how you feel the trip was arranged. Just because you reserved the room with your points does not necessarily make you the host/hostess and hence, responsible for paying for their portion of the accomodations. It really depends on the nature of the original discussion and the relationship you have with these other people. There is nothing wrong with going on trips with friends and splitting the costs. The fact that you reserved the rooms with your points does not make it any more your responsibiity. Nor does it make you less of a friend or less generous in the instance you do not assume the full share of the costs. Especially, if this is what you had discussed (loosely) prior to the trip. Heck, even paying what the rental would have cost on a per point basis would save a significant amount of money for your friends.
Now the scenario where you invite someone to come with you as your guest - which you would clearly state prior to the trip, is another story. Or if you invite someone who would otherwise not have gone/could not have gone without your financial help, is also an exception.
Of course, as you mention, it feels difficult, since the trip has come and gone. Looking at it from the other perspective, I wonder why you have not been contacted by the other party saying "Hey, what do we owe you?" since clearly this was discussed, at least in loose terms, prior to the trip. That could just be me though, because I would never expect someone to cover the expense of my accomodations unless they won the lottery! I do not expect money from my friends and would feel ashamed/duplicitous to take it in most circumstances.
This certainly can be a difficult topic. An analogy would be dinner. We occasionally go out to dinner with our local friends - and we always split the costs of the dinner. This is regardless of who contacted the other party to go out. Why would we expect them to pay for our meal or vice versa? However, when we have friends come into town that we have not seen in awhile, we insist on taking them out and grab the check. If they argue we always say that they are in our town and it is our pleasure to host. When I had my birthday party at a restaurant and invited all of our friends to join us, of course we paid the bill - we were the hosts. I bring this up to illustrate that it is how the dinner (in your case -trip) is arranged and the nature of the relationship between parties. As you mentioned in your post, you did not expect to host and you discussed this with your friends. There is nothing wrong with that! Give them a call. I hope you work it out.
Good luck !