Click on 'Manage matter' at the top right hand of your screen and select 'Participants' on the drop down.
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Underneath each role is box with a button to 'Add new organisation'.
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Type in the name of the organisation you want to add to the role. You can add a description is needed.
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Click 'Create organisation' to create the organisation.
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If you want to restrict who can see the organisation, click on the grey 'Viewable by' button. For more information on this feature, please see the article 'Hiding organisations'.
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The organisation created will now appear under the relevant role. At the bottom of the box for each organisation, there's an option to add the email address of the users acting for the organisation.
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Hover over the organisation and click on the "three dots" then select "Delete organisation".