You'll need to request access from an Administrator ( if the account is private) before you can post a message/content. You'll also need to create a Memories account before our security settings will allow you to post. The Administrator will add contributors along with the corresponding content permissions that the Administrator may impose.
But if the Memories Timeline is public, and a link was shared with you being one of the Contributors, you can immediately view the content of the Timeline but won't be able to participate unless, you already have an account or have created a Memories account. We require this for security reasons, and to avoid spam and inappropriate content.
Contributors can add content by just clicking "Add Memory" from the bottom left section of the Timeline.