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

• Tap on the three lines in the top left corner
• The left navigation tray opens up
• Select Group/Subgroup
• Click on Add members at the bottom
• Click Add to <Group Name>
• Click the Member/Administrator in 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 the three lines in the top left corner
• The left navigation tray opens up
• Click on the Group
• Click the Settings Icon in the top right
• Click Edit Settings
• Click on Bulk Import
• Upload the excel spreadsheet with addresses. 
• Once you have uploaded the number of contacts imported would be displayed.  
• Click on Invite.