You can use a ROOM or a GROUP message in Google Chat to chat with two or more people. To help you decide which to use, the table below describes the differences between group messages and rooms.

 Group messagesRooms
Who can chatAfter starting the chat, can't add or remove anyoneCan add and remove anyone at any time
ConversationsSingle unthreaded conversationMultiple, threaded conversations
NameList of other members' names, e.g. "Alana, Michelle, Zorica"Room creator chooses name, e.g. "IS Team"
NotificationsMembers are notified for every messageUsers are notified for conversations they're in, or when @mentioned
LeavingMembers can't leave, but can hide the chat in the sidebarMembers can leave and rejoin
Multiple with same usersCan't have two group messages with same set of peopleCan have two or more rooms with same set of people
External usersCan't include people outside your organizationCan include people outside your organization 


Here are some examples to help choose between a Group Message and Room.

Organizing lunch plans - Group Message. All people will be notified of all messages so that any venue changes, time changes will notify everyone.

One-off message to two people - Group Message. When adding someone to a room, it'll send them an email to join. If you need to quickly inform people that their package has arrived, or they left a meeting room a mess, a Group Message would work best

Culinary Team at Bria site -  Room. Allows for relevant conversation threads to be grouped together, and only people in the discussion will be notified of any updates. If Chef posts about a recipe and only the sous chefs need to discuss, not everyone will be notified of any replies. 

Project Group - Room. Multiple aspects of a project can be discussed concurrently, with each conversation separated by thread.  All team members can see history of discussion and can easily find info they need.  

Office Group - Room. There's nothing more annoying than a million notifications for donuts at the office when you're not there. A Room will notify everyone a new thread was started ("Donuts in the kitchen!"), and all replies will only notify those who are in on the discussion. You'll still miss out on the donuts, but you won't be notified each time a colleague replies "thanks!" or "OMG, those were so good!"

IS Team - Room. We have several threads every day, discussing different sites and specific projects. Relevant replies are grouped together, allowing the team to be informed about what is happening. When new information is added later, it becomes part of the same conversation thread, which preserves context.