As you now, with Teams you can initiate chat conversations with your colleagues and/or external users (if permitted).
Well, the chat capability has been extended to now allow you start a chat conversation using groups (distribution groups, mail enabled security groups or Office 365 Groups).
To start a conversation using one of this group type, just start a new conversation and search for the group (either name or email address) you want to start the conversation
The rollout is currently in progress and expected to be completed by late October.