This plugin allows you to add RSVP and guestlist functionality. Guests can leave custom messages for the planners. Uses Google Docs spreadsheets.
Check the homepage for a lot of comments and responses about how to fiddle with this plugin, we have had a bit of discussion and help from other users.
Note that you are required to have PHP version 5. Sorry, the only solution right now is to use that version of PHP.
If you cannot change the text in the options page, you can change it in the source code (by editting wp-gustlist-plugin.php). This is not a very safe thing to do, but you could search the code for the words you want to change and fiddle around with it. This take familiarity with HTML and a little PHP.