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

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

You can also send a group invite:

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

You can also bulk invite the members to a group. 

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