You can thank your contributors in 2 different ways:
1. By posting on the "News" Tab of your project, for a collective note
This way, your message will be publicly visible on your project page. All contributors and everyone who is following your project will then be notified by email that you posted a news.
This is an ideal solution if you want to thank your backers collectively.
2. By using our internal messaging service, for a more personal note
These messages are private and only visible by the person you are writing to. To thank your contributors personally, go on your project page, then click on the 'KissBankers' tab : there you may choose to contact all contributors at once, or only a part of them (using filters such as the rewards they chose, the date, etc).