How to write post very simple

I have made a wordpress-site (twenty twelve) for a client and my problem is that there are a number of persons that are going to write posts on the site but I don ‘t want them to get access to the admin-tools, only the blogg. How do I do that? The persons are not computer-friendly and on the site before (not wp) they wrote i a guestbook and that they could manage. But Wp doesn’t have a gustbook or??

Thanks in advance!
Bee