Sending a message to a group of contacts is as easy as creating a flow. Once you've created your flow, add an action containing the message you'd like to send and then select your target group from the contacts dropdown in the flow start node.
Let's walk through the steps.
1. First click on the Flows tab in the navigation bar.
2. From the Flows page, click on "Create Flow". Name your flow and leave the other settings as their defaults, clicking "Create" when done.
3. You will be taken to the flow editor for your new flow. Click on the "Create Message" button.
4. In the dialog that appears, enter the text of the message you want to send. Make sure to use the "Send the Contact a Message" action. Once complete click "Ok".
5. Now click on the "Start Flow" button. In the dialog that appears you can select the groups you want to receive your message.
Once your group is selected, check the settings you'd like to apply and click 'Ok' to start the flow.
"Restart contacts who have already entered this flow" means any contact who has already passed through the flow will be restarted. This is important if you want to collect new data from the same contact.
"Interrupt contacts currently active in flows" means that if a contact is in the middle of another flow, they will be automatically exited from that other flow and started in this one. Note that if you do that, the contact will not be automatically restarted in the other flow.
Checking both settings will ensure your group's contacts receive the message regardless of their message history or activity in other flows.
More on starting a flow here.