The introduction text can be added in multiple ways in system. For most customers the introduction text is added through Administration->Custom Texts->Advertisement - Introduction Text. All users who have access to Administration have access to edit the introduction text.
It's important to stand on the correct department when you enter Administration to perform actions. If the introduction text is added on root department then you must make sure to enter Administration through root department when editing the text.
The text should preferrably be written directly into the editor so that you're inserting plain text. If you would have to copy and paste text into the editor you should only copy and paste text that is in plain text format.