Also, if after selecting the AddIn, Excel prompts to "Copy AddIn to default AddIn folder", select No. Or else excel will copy it to your local AddIn folder, and it will be using the local copy ISO the network copy.
Yes, every user will have to select the add-in once. From thereafter, when they open Excel, the add-in should be selected by default.
No problems accessing simultaneously, except when modifications to the code were needed, as I noted above, you'll have to ask them to exit excel. Jon Hawkins