As Nate said, you can enable Discobot’s welcome message and add the information into that.
The setting to enable the welcome message is Disable Discourse Narrative Bot welcome post. You can customize the message like any text Customize text in Discourse. The relevant keys are discourse_narrative_bot.new_user_narrative.hello.title for the title of the message and discourse_narrative_bot.new_user_narrative.hello.message and discourse_narrative_bot.new_user_narrative.bookmark.instructions for the body. By default Discobot also offers a tutorial in that message where you can learn about Discourse features like oneboxes, bookmarks, and flags.
With the Discourse Narrative Bot welcome post type setting, you can change it to a different welcome message that doesn’t offer a tutorial. Then the site texts you might want to customize are system_messages.welcome_user.subject_template for the title and system_messages.welcome_user.text_body_template for the body. The %{new_user_tips} placeholder is replaced by system_messages.usage_tips.text_body_template, so you can customize that too.
There is also an automated message when the user reaches trust level 1 (Customizing trust level promotion messages for new users). But I think that’s too late for your use case.
For a more personal approach, you can use the automation plugin to trigger a personal message when a user is added to the trust level 0 group.