Allow short message exchanges between a list of approved users?

We have a group of a few dozen bicyclists. I want to be able to implement a “ride exchange” feature within the WordPress “blog” that serves as our website. I’d like to allow anyone in the group to post a short notice saying “I’m going on thus-and-such ride tomorrow, anyone want to ride with me?” If possible, I want to restrict access to a specified list of people to avoid spam.

Traffic will be low so I can do manual management of things like removing out-of-date message, but timeliness is important and I don’t want to have to moderate each message.

I don’t care if the feature is terribly slick as long as it’s functional. It could be something as simple as a designated page and directions telling people to leave their messages in the form of comments, EXCEPT for the spam issue. I’d prefer not to install (yet more) plugins unless there’s one that’s really a good tool for this job.

Another possibility is a Twitter account with a widget to display recent posts… but I’m having some issues getting that to work.

Another is to install a phpBB forum or something of the sort.

Just some homely suggestions for workable ways to get the job done.