We have reintroduced the ability for teammates to add and remove conversation participants in the Inbox without having to send a reply for the changes to take effect.
In the '3 dots' menu on the conversation header, click on Manage Participants. You can also find this option by using Cmd+K.
After adding and removing participants through the modal your changes will take place straight away.
In the composer you can still add and remove participants by changing them in the 'To' field. You still need to send a reply for changes in that field to take effect.