If your SME Contact has multiple entities, you must create an Engagement for each entity using the same email address. Just follow the regular Engagement invitation process available here, use the same contact details for each entity, just change the company name and External SME ID (if applicable).
The SME Contact will see all of the engagements they have been invited to, listed in their DataShare portal the next time they sign in to DataShare.
If the SME Contact has already signed up, the new engagement(s) will be added to their existing engagement list.
The SME contact will be able to select additional engagements by clicking on the tabs on the top left.