If it's an event open to the public, like a bike ride, there's not much one can do other than what's been mentioned re: "see you at the after event" or being vague about whether you're going. In this case, I don't think the OP and her friend were rude for talking about it. And Mr. Tag Along strikes me as more clueless than rude.
Not everyone is going to be invited to every single event. For example, I have a great aunt who hosts many family events. She is a gracious, generous person, so in addition to her sons, she includes extended family, such as myself and another aunt. She also includes a cousin from another side of the family who isn't related to her. This cousin has become possessive of our great aunt and doesn't want anyone to do anything without her, even her sons!! In contrast, I appreciate that she & her sons are willing to share their family with us, so on the rare occasions they want to gather just with themselves (due to space reasons), I'm glad they're enjoying their time together and I don't expect an invite. The possessive cousin is the one I blocked on Facebook because of her lack of respect for boundaries. One of the sons (also one of my cousins) posted about his weekend which included one of these mom & sons dinner. Of course possessive cousin had to make an issue of it. I just liked his status, that he had such a nice time with his mom & brothers.
I don't feel he should have to be secretive about it when he and his mom/brothers do mother-son, brother-brother things. The exception is when she's been excluded because of her behavior. Several people can't stand the way she dominates conversations, regularly disregards boundaries & puts other people down (trying to curry favor with my aunt, but not realizing that's the absolute worst way to go about it) and don't want her around for certain events. I've been invited to some of those and I've made every effort to make sure I didn't inadvertently say something. I even set up another photo sharing account to share the photos. Since she has access to my usual account, I didn't want her to see photos to the event she was excluded from.
Another example of not being expected to be invited to everything is a good friend of mine's family. I've been cut off by my mother & brother and my father is dead. In addition to my extended family, some of my friends have really stepped up to include me in their families. For example, I spent both Christmas & New Year's Eve with this friend and her family. She & her sister joke around that I'm their long lost sister. I appreciate the invites & inclusion and I don't expect to be invited to everything. The 2 sisters do things on their own and I don't expect to be invited, nor do I expect them to keep it secret from me. They are both on my Facebook list and they sometimes talk about the activities. If I'm not specifically invited, I figure it's something they want to do on their own. I don't invite myself along and I don't feel hurt by it.
If anything, I consider myself lucky to have my extended family & friends in my life. If they occasionally want to do family of origin type of things, I understand and don't feel excluded. It would be weirder to me not to have them talk about it. I wouldn't want them to feel like they're walking on eggshells.