Linking Tables Together
I would recommend adding another table, StreetLightGrouping. This table should have two columns, StreetLightGroupId and StreetLightId.  All the StreetLights under one StreetLightGroupId are part of the same group.

Also, if a StreetLight can belong to a maximum of one group, a unique constraint can be established on the StreetLightId column.