To add additional members to a group, go to the package of the group which you would like to add the member to:
You should now click on the row of one of the group member to bring up the groups allocation:
Click on the 'Member Info' tab to view the list of members in the group and select the '+ Add member' button at the bottom of the page:
Now you can add a new or existing contact to the group. Once your happy with all the details, make sure your use the 'Save' button at the bottom of the page to add the member to the group.
You can also remove members from a group using the 'Remove' button's adjacent to each member on the 'Member info' tab.