When Myrmex detects that a unique user (same email address) belongs to several organisations, it enables that user to switch immediately from the account of one organisation to the other ones.
Such users will have a small down arrow appearing next to the organisation name in the main menu:
Clicking on the arrow will unfold a dropdown menu with the list of organisations the user belongs to. Simply picks the account you want and Myrmex will switch you to the account of that organisation.
Next time you log in, Myrmex will per default put you on the last organisation you have used.