Chat with members from your distribution lists, M365 groups, or mail-security groups in Microsoft Teams

You can start a group conversation with people in your distribution list, Microsoft 365 group, or mail-enabled security group. The group can have up to 250 members. 

1. Go to your list of chats and channels, and select New message .
2. Next to To, type an existing group name. 
3. Select from a distribution list, M365 group, or mail-security group to add new chat members to the existing group chat in Microsoft Teams.