Help+Manual allows multiple users to edit the same project at the same time when both are on the same local network with its integrated multi-user editing capabilities. Users in different locations should use version control for collaboration (see further below).
The only restriction in local multi-user editing is that it is not possible for two authors to edit the same topic simultaneously. When one author is editing a topic, a second author accessing the topic can view it but not edit it. A message will be displayed saying that the topic is read-only because it is in use.
Multi-user editing works directly "out of the box". You do not need to do any server configuration. There are no special settings and you don't need a database or additional external software, either locally or on the server. You just need to store your project in uncompressed XML (.hmxp) format on a drive that all your authors can access and start editing.
Refresh your project regularly! While collaborating on the same project always click on Refresh Project in the Project menu whenever you switch to a new topic to make sure that you see the current version of the Table of Contents and Project Files.