You may want a Space that contains internal team members and external persons such as contractors or clients.

In order to do this when you create the space you must enable it for external users. 

NOTE you cannot edit an existing Google Chat Space once created to allow external users. 

Start at chat.google.com or use the sidebar on the left of gmail.com 

1. Click on create a space
2. New Space
3. Click on Advanced
4. Check box on "Allow people outside your organization to join"
5. Then add external user email addresses
6. Click on create

Note that the users you are adding will need a Google Account. If they do not you can point them towards this article showing how to enable any email address as a Google Account (and you dont need an @gmail.com).