Do I have to create an account to post content?
In order to post content or contribute to a Timeline you need to create a Memories account. If the Timeline is private, an Administrator must invite you as a contributor in order to post content/memories. Refer to this article Signing up and getting started if you are wanting to sign up and create an account.
However, for a Public Timeline, once Contributors have been set-up to participate in a Timeline by the Administrator, they can immediately view the content of the Timeline but won't be able to participate unless they have already created an account, please refer to the process on How to add a memory to a Timeline?
Contributors can add content by just clicking "+ Memory" from the left bottom section of the Timeline.