Go to your group at  https://groups.google.com/  

Click on the group you are wanting to add members to then follow the instructions below


To directly add people to your group:

  1. Click on the Manage button below the group name. A list of items appears on the left-hand side of the screen.
  2. Click on Members. Several items are listed.
  3. Click on Directly add members. The Add/Invite members screen appears.
  4. Click the Add members directly tab. The Add members directly panel is displayed.
  5. Type the email addresses of the people to add, separated by commas, in the ‘Enter email address to add as members’ field.
  6. In order to directly add members, you're required to type a welcome message, which you can type in the “Write a welcome message” field.
  7. Click on one of the options under “Email subscription options” to set how users will use email to interact with the group.
  8. Click the Add button to directly add the people to your group.