try when only having selected the ships you want to add...
it's different than most other rts, because in most of those you have the following:
select a ship, shift+group number -> you now have the ship selected and additionally the group as well, though the group still doesn't have the ship in it, you would need to redefine the group (strg+number) to change it.
in sins it's rather:
select a ship, shift+group number -> ship is added to group, though you still only have the ship selected, if you want to have the full group selected you now have to press the group number...
i guess they should have an option to use the behavior most other rts are using as well... i kind of confuse that quite often as well, so instead of having 2 groups selected i end up redefining both groups, so they are completely different from what i expected...