You can think of following options.
- If those customer ID's are added to AD and you have SOI (EEM) and AD integration then your customers can access SOI URL and login to it.
But this is not feasible solution as your client will have lot of own ID's to access portal.
- Create a 'Unique' ID for each customer companies and after logging to your company portal they can access SOI with this 'Unique' ID.
So the point is, if the user is not in EEM and added to any group in SOI, you can not launch SOI portal.