We have the same case and  what we did was setup supplier, product and customer categories for each company.  Then setup roles on each users account specifying which categories they should see.  If you don't want customization (who does) then this is good solution as can do it yourself.

Adding 1 specific field in a screen and table has little side effectbut at worst you can use the field related to the product line to avoid creating new one (CFGLIN). This is relevant if you never use the configurator.

Another solution might be to set the ITM object with a new specific field as access code field (in the general tab). I have never tested this approach but it might more convenient if you have a product that can be shared between 2 companies.

Adding 1 specific field in a screen and table has little side effectbut at worst you can use the field related to the product line to avoid creating new one (CFGLIN). This is relevant if you never use the configurator.

Another solution might be to set the ITM object with a new specific field as access code field (in the general tab). I have never tested this approach but it might more convenient if you have a product that can be shared between 2 companies.

adding new field and applying access codes is good too but not sure if it will limit what you seel in the selection.  We do have issue with the roles on the SO side where when we go to select product on the line the list will display all products however if jump to the product screen then the user can only see the products they are allowed to see based on the role/category commbination.   Appears the ITS screen does not have the category field which however on the PO side the user will only see those products they are allowed.  AGain- not sure why Sage is not consistant with things like ths.   in any case there should be flexibility with this especially for clients with multi-legisllation and multiple financial sites.  :-)

Product selection on sales is based on ITS object and not ITM. The technical reason is simple; only sold product should appears in the product selection of sales documents.

You may used restricted price to prevent this to happen or again to redo the customization one more time but using the ITS object.

We created fields on the product table and developed a filter inside the selection script. Hard, ugly, but practical to manage.

It seems this topic was already adressed in sage city: sagecity.na.sage.com/.../how-to-restrict-products-by-company