To create a Group all you have to do is go to your organization's App or Web URL and select Messaging. From there just click the "+" button in the bottom right-hand corner.
To add people to that Group, simply enter their phone number, email, or user name if they already have an account set up! Those invites will be sent out by email if you opted for email or existing user, or by text message if you’ve entered their phone number.
Note! Inviting by phone number is currently only available for U.S. and Canadian phone numbers.
If you accidentally invite someone to one of your Groups or need to remove a participant for any reason you can quickly do that through the settings on that Group. Just scroll down to the user you’d like to remove and press the x on the right side of their name!