The email message says “Your friend invited you to twitter!”
Sadly, it isn’t true. The sender isn’t twitter at all, but a spambot, sending you a virus-infected file, in zip format.
So delete the message and be sure it is purged from your mailbox.
Here is one of the messages, so you can see what it looks like:
Twitter is a service for friends, family, and co-workers to communicate and stay connected through the exchange of quick, frequent answers to one simple question:
What are you doing?
To join or to see who invited you, check the attachment.
Popularity: 98% [?]