Sorry to contradict you, but to me an addin IS very very similar to a
workbook, I mean it must contain at least one worksheet and especially can
contain data: this data can be changed at runtime (through userforms for
example), and if you need to find back that data in a further session then
you need to save the addin, programatically. (Beware, if Excel is not
instructed to save the addin, then Excel won't warn the user that some
unsaved data will be lost on quitting!).
Also because an addin is quite like an hidden workbook, simultaneous write
access won't work (never tried to make an addin from a shared workbook
though). Use Access if you need to achieve that.