Lets say if it spreads from single member to friends with bubble effect; then having name of the member(preferably genuine name otherwise userid/handle) in subject line can help in long way.
e.g. morbidkk wants to keep up with you on Twitter
I personally dont even read invitation mails from strangers.
1) so avoid generic subject line. - A friend invites you to xyz
2) Also check shelfari invite mail; which is very impressive as it just introuduces everything what can be done on shelfari in small but concise mail.
The single most important thing is to get the inviting party's name in the subject line. A personal note from the inviting party is golden, rather than a form mail. If you have to do a form mail, keep it brutally short.
I personally dont even read invitation mails from strangers.
1) so avoid generic subject line. - A friend invites you to xyz 2) Also check shelfari invite mail; which is very impressive as it just introuduces everything what can be done on shelfari in small but concise mail.