Sending a message to a group of contacts is as easy as creating a flow. Once you've created your flow, add a Send Message action containing the message you'd like to send and then select your target group from the contacts dropdown in the flow start modal:
Once your group is selected, check the settings you'd like to apply and click 'Ok' to start the 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