1. What does it mean?
단톡 is a short form of 단체 톡, often used for a group conversation on a messaging service. 단톡방 refers to the room itself, such as a family, class, or project chat.
Also written or said as 단톡방, 단체톡, dantok
2. The nuance
It is not necessarily an open chat anyone can join. A workplace 단톡 can contain formal communication even though the word naming it is informal.
3. Examples
“이번 주 준비물은 단톡 공지에 적어 뒀어.”
I put this week's supplies in the group-chat notice.
“단톡 알림이 많아서 잠시 무음으로 해 놨어.”
There were so many group-chat notifications that I muted it for a while.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.