'External guests' able to join community?

Hi, 

We have merged two companies, and until we have one tenant, we are having to add one organisation as guests to one tenant instance of Viva Engage. 

This means creating a new 'All Company' community for them - everyone will be added. 

We are also looking at more optional communities (ie an AI community) - is there a way that the 'external guests' can choose to join this community? What would be the best way instead of adding everyone into it?