1) Create a text box for keeping track of usernames.
2) Create a Rule in Form Options that adds userName() to it for the first uer.
3) Create a condition that says if the text box is not empty and does not contain userName(), then write the current contents of the textbox plus the current userName() back to the textbox with a delimiter of some sort (e.g. ";").
That sounds like it will work as long as it's ok to have a running text box of usernames with delimiters.