Obviously I wasn't there but from what you wrote it sounds like you may be overreacting. Are you sure they meant to slight you? Maybe they just wanted to go by themselves. You wouldn't want "well we had this great day planned, but there's this girl and she's nice, we should invite her... sigh... I guess we'll do that, we'll have to reschedule our fun day for some other time." They shouldn't feel obligated even if they have the best intentions.
Obviously this is out of context, but maybe you should just invite them to ride with you sometime? Build up a relationship.
If you think maybe they meant to slight you I'd simply ask how their trail ride was and say you'd love to go with them sometime.. extend the olive branch. If they keep on doing it I would ask them if they prefer riding by themselves or if they have some issue with you because you would like to go riding with them, but feel like they don't want you along.
So again, out of context, but just make sure you have the right impression and don't over react. Honestly I prefer to ride by myself and if I had a date with a friend, yeah I might probably invite you (aka whoever) to be nice, or because my friend said to, but I'd be more comfortable without you, I'm just an introverted person.. Now if someone invited me, to ride with them, I would be more interested and be thinking more about building a relationship and maybe having a new riding buddy instead of feeling put out about a change of plans. Guess I'm trying to say that some people may simply prefer to ride by themselves (or with their "usual group") then with someone else, and that it doesn't necessarily mean they don't like you, or even that they don't want to ride with you, maybe they didn't think of it. Who knows.
Sorry, I'm rambling. But I would put yourself out there as "hi, I want to be a friend, and definitely want to go riding with whoever's available" and again, invite people to ride with you. Don't be afraid to bridge the gap yourself, some people need that!
I'm off to bed and will check in and hopefully make more sense tomorrow :)