The transcript is the full thread. Open it from a person's timeline and use the box at the bottom to send a text from your company number.
Hover a message and choose Reply to quote it. On an email thread, that control is Reply by email.
If they are already in a conversation with the assistant, the box says so. Wait until that wraps up so your text does not land in the middle of it.
How to use it is in Read a transcript.