I have the same issue with the club here!

My 2 cents... yes you should make your own decisions about people, but if the co-worker who "warned" you is trustworthy and you know that they have your best intrest in mind, it's worth keeping your guard up with this other guy. Thus far I have found that some of the older guys are a great resource, and some just keep hitting on me no matter what. I found that the younger male cyclists tend to be fast, so if you think you can hang with an A pace group go find the local racing team rides and hang with them as long as you can. Some have serious ego issues, but most are happy to see a woman out there...