Oh, and editing again after reading your post about being passed: that guy may be doing a "low-key" ride (but wiht a little sprint because they ... ahem, we

...*do* that to pass people... bet you dollars to donuts he slowed down right after be blew by you). YES, the bike and especially the underinflated 38 tires make a HUGE difference.
When I switched from 35 to 28 tires, it was an instant 2 mph.
14 mph is pretty fast to do on a hybrid with 38 tires. THe other little piece of reality is that usually (not always) if somebody says they go 14... well, they do. Except whenthey're going 15 or 17. And ... the stark reality is they tend to say "you'll be fine," because it's really hard to tell somebody "oh, I don't think you'll be able to keep up with us."
- and did the person you were talking to know the bike you are riding?
"Low Key" is a totally, totally relative term.
I have years of experience being an I-can-go-the-distance hybrid rider, but could not keep up speed wise with the guys on the road bikes - and that was with 32's and 28, not 38s. I ended up forming my own group... and getting a lot stronger on my own, and then getting a much lighter, faster hybrid (which is wonderful because it still looks like a hybrid but I *can* average 17 or 18 on it).
I would try to find a way to inconspicuously size up their group before making an appearance. If they look like those guys in the Hot Mail thread, especially with the fancy gear, then the odds are it'll be an uneven match. THey'll prob'ly wait up for you (but not necessarily). And... sometimes the "12 mile" trip part is bogus too - especially a show & go because by definition there are no rules.
On the third hand, though... if they're like my buddies in Richmond... not only will they ride with you at your speed, they will do it so smoothly- you won't ever be at the end - that you will not even realize they usually go faster until later. It really all depends who's out there.