On goodreads, they can (even without joining) visit the book pages of some of their favorites or new releases and scroll all the way to the bottom to see which groups are discussing currently or recently. Many gr groups do a book of the month discussion which will get most active participation but will also offer "buddy reads" where one or more members join in less formally. Glancing through or lurking a few discussions should give them a feel for which groups fit.
Booklikes' groups (discussion rooms, now buried in dashboard menu that doesn't work for me in this new redesign) are relatively new and need to beef up book discussion features -- but something to watch when does get more active.
Some publisher websites do community reads. (Like http://dawbooks.livejournal.com for SF/fantasy genre)