Adding a node to a service group:

Look if you can see the new node via GAB (gabconfig -a). If the node is visible:

GUI:
right click on the service group -> view -> properties view -> show all attributes -> SystemList (edit button) -> plus sign -> enter node name and priority for this node

Then put this new node also in the AutoStartList if you want just the same way as modifying the SystemList attribute.

or

CLI: 
hagrp -modify  <service_group>  SystemList  -add  <new_node_name>  <priority>
hagrp -modify  <service_group>  AutoStartList  <old_list>  <new_node_name>

The node name ist the same one as in llttab. Normally.

Good Luck
Roger