This use to be a great way to meet new people and talk about topics that are of interest to you. I was in about 10 to 15 groups at any times about music, bands and even tv shows that I like. All of this changed when Yahoo bought eGroups and now there are a lot of adult related areas and people are spamming the groups.
But how it works is just like a mailing list (in fact that is what it is with a few exceptions). You can also read your messages online (in your group area) and post pictures and polls and get to know a lot of interesting people.
Now since Yahoo bought out eGroups they are more like Yahoo Clubs than the fun communities that they were before the buy out. There are ads attached to every message if you get them in e-mail form (which is the easiest way to go if you are in many groups).
You do need a yahoo id to join the groups now - which before you just needed an e-mail address but it’s okay because you can use your Yahoo ID for everything else that Yahoo owns as well.
All in all the groups are still simple to use and a great way to meet new people.