You can add more members to your group by following these steps:

• Tap on All Groups at the Top left corner
• The left navigation tray opens up
• Click on Invite to Group at the top
• Click on the Name of the Group in the Invite to Which Group? screen
• Click the Member / Administrator in For Which Role? screen
• Select one of the three invite options in How to Invite? screen

You can also send a group invite:

• Tap on All Groups at the Top left corner
• The left navigation tray opens up
• Select the name of the group under Groups label.
• Tap on Members.
• Click on Members +.
• Click on Invite Members.
• Click the Member / Administrator in the For Which Role? screen
• Select one of the three invite options in How to Invite? screen

You can also bulk invite the members to a group. 

• Tap on All Groups at the Top left corner
• The left navigation tray opens up
• To add members in a group:
• Click on Members.
• Click on Members+.
• Choose between Invite members and Create Sub - Groups.
• Click on Invite members.
• Click on Import. 
• Upload the excel spreadsheet with addresses. 
• Once you have uploaded the number of contacts imported would be displayed.  
• Click on Invite.