All text in .threads is stored in language files. The language file it's stored in is related to the script that is actually running. In this case the welcome message is being added when a user is added, ubb=adduser, so that's where you'll find the text for this.
As it is, this is what it says:
Welcome to our forums! Please take a moment to review and update your profile and preferences to take full advantage of our features. You can do this by clicking on "Edit Profile" and "Edit Preferences" in the Forum Navigation island.
This topic has no other participants. It is simply a welcome message to thank you for joining our community.
That can be changed as needed by editing the language file. Maybe it needs to be edited to be more clear, the way it sounds right now is maybe misleading that it sounds like there are no other participants besides you and the person who sent it.
