Within Teamwork Chat, you can add additional users to a conversation or channel after it has been created.
When you are viewing the individual conversation or channel, go to the details pane to the right of the conversation thread and select the Members subsection.
Select + Add Members to invite people to the conversation or channel.
Once you have selected the user(s), click Add Members to add them to the conversation.
The user(s) added will now have access to all messages within the conversation or channel
Note: Only the creator of the conversation and owner company administrators can manage the people in the conversation or channel.
For more information, see: Creating a Conversation or Channel