You can import a spreadsheet of contacts into a contact group using the import tool. Using the side menu, click on Configuration > Contact Groups.
Create a new group by clicking on 'Add New' or use an existing group by clicking on 'Import Contacts'.
You will be taken to a new screen where you can download the import template. This is an important step, without the template, your import will not work. All fields within the template that are in bold are mandatory.
Download the import and enter/copy and paste in the details in each column. Save your file.
Click on Choose File and search for your import. Click on Import once you have selected your file.
Another screen will appear once this has occurred for you to review your import. This review screen will show you three columns from your import - Name (first name), Email and Mobile Phone.
Select whether these are new contacts or modified and 'Save Contacts'. This will shoot you back to the screen of all groups you have listed. Click on 'Open Group' to view your newly added contacts - you can edit, or remove a contact from this screen.