As the blog's owner and administrator, you would like other people (friends, family, colleagues…) to be able to write on your blog from their own account?
To do so, all you need to do is click Writers in the left-hand administration menu.
Type the Gandi login for the user you wish to invite in the Invite a new writer field.
Once you have typed the login and clicked the invite button, an interface will ask you which permissions you wish to grant to the new writer.
Here is a quick description of possible permissions:
When you access the Writers interface, the users who have access to your blog will be displayed here. Just click change permissions by their user name to modify the permissions for the selected user.
To delete access to your blog to a given user, change his permissions and remove them all, and Save. The user will then have disappeared from your blog's invited writer list.