It's very straightforward... you sign up to the group(s) of your interest and chose how you want the e-mails delivered ( individual mails, daily digest or read online only at the group's page on the Yahoogroups website ). If you find out any solid gen yourself or see something of interest to the group, post it by e-mail and it'll automatically go to every member of the group.
Do be sure to read the rules on each group - some are very strict while some allow anything as long as it is at least related to the group topic.
I am a member of about a dozen such groups ( plus a few non-railway ones ) and I own and moderate five* of them.
*Including three of the non-railway ones.