join -t , -j1 N1 -j2 N2 F1 F2 would take two comma separated files, F1 and F2, sorted on the email field (N1 and N2), and join them. See the man pages mentioned before for other flags that can be used.

It has been said that you can build a pretty complete DB with just the common unix commands (grep, sed, join, sort, uniq). Not that it would be overly fast, but it could be done. Since perl incorporates parts of each of these utilities, I would probably reach for a perl solution for anything beyond a trivial need.