So don't do it.
In fact, be wary of any "social networking" site that asks you to do this. As he recommends, check the net and see what other people have to say about a site before signing up.
I personally received three copies of my friend's invite to join Quechup, so I was suspicious already.
ETA: and when I tried to cancel my membership it says "Unable to log deregistration details." -WTF? lame lame lame lame lame.
(second attempt to cancel, using one of the set causes instead of 'other' appears to have gone through.)