Within Teamwork Chat, you can choose to automatically add new users to a particular channel by default.
To make an existing conversation/channel into a default channel, click on the room settings in the top right corner.
In a channel, click the 'Edit channel' option. In a conversation, you will need to convert it to a channel first, clicking the 'Convert conversation to channel' option.
In the create/edit modal, select the 'Automatically add new users to this channel' option:
Once selected, every new member will be added to this channel as default.
For more information see: Creating a conversation or channel