Using Joiin's 'Fine Grained' user controls, you can restrict invited users to specific companies or groups of companies connected to your Joiin subscription.
You will first need to create a 'Group' of companies:
1. Head over to the Companies page on the left-hand menu
2. Select the Group structure and add the group structure
3. Create a new group structure with the company/ companies you would like the user to have access to and save
4. Go to the users page via the settings cog and click invite user (For existing users, press the pencil icon next to their name in the invited list)
5. Scroll down to the access and select the group you would like the new user to have access to and add user.