firstname.lastname@example.org is for general announcements. It is here that events organized by the Round are advertised, as well as the occasional important administrative message. This is a moderated list; postings will be forwarded to the list if considered relevant.
If you are interested in the Round or its legendary ceilidhs, you should definitely be on this list! To join, go to lists.cam.ac.uk/mailman/listinfo/soc-round. The list of members is kept private and you will not be added to other lists without your consent. Use the same link if you want to be taken off the list. Note that there is no need to explicitly request to be taken off if you are leaving the university as you will be removed when your account is cancelled, but of course you may want to have messages sent to your new email address instead!
email@example.com is mostly unmoderated (although to combat spam messages from addresses outside Cambridge University which aren't on the list will be forwarded for moderation in the same way as soc-round). It is intended for regular Round goers (although anyone is free to join) and is where info on good gigs, random requests for help, info on weekly Round meetings, etc. should be posted. Messages sent to soc-round which are not considered suitable may be forwarded to soc-round-chat instead. To join or leave the list go to lists.cam.ac.uk/mailman/listinfo/soc-round-chat.
I know that's a lot of addresses... don't worry, if you send to the wrong one it'll get forwarded to the right one fairly quickly, and roughly the same people are behind most of the administrative ones anyway. Just remember to sign up!