You don't give the signin details to share your family tree. You click on the person you want to share it with and press the "In vote *name* to share tree" button. On that screen you can uncheck the "Allow *name* to edit family" box. You can send the invitation to one family member or copy the link at the bottom and email the link to several people at once.
NOTE: there is currently no way to revoke the ability to view or edit the family tree. Even if you delete the tree, others with the link will still have access to it.
Also, you might want to download a backup of you tree before giving anyone edit privileges.