I already thought of using the environmental variable. However as it seems
Excel saves the path to functions in add-ins absolute, not relatively in the
file. So the problem is not returning the environmental variable in the VBA
code, but that the second user's Excel can not find the function in the
add-in at all. Even when the add-in is loaded (but saved in another
location). Saving the add-in in the user's folder or the excel standard
folder also returned the same problem.

How is this problem being solved in the standard Excel add-ins Eurotool, or
Solver? Eurovonvert() for example definitely work across different users and
Excel versions (which means different folders of the file)