Hello and welcome to Openhab. And yes, groups can be a little tricky to grasp at first. Yes, you can do this with rules if you prefer. But if you want a Group Switch, what you want to do is the following:
Set up a Group Switch in your items file, and then assign your lights or outlets to that group. As an example, this what I have for my Family Room Lights.
Group:Switch:OR(ON, OFF) Family_Lights "All Family Lights [(%d)]" <light> (All)
Color FF_Light_FamilyCF_1 "Ceiling Fan 1" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="hue:0210:00178824e7e9:13:color" }
Color FF_Light_FamilyCF_2 "Ceiling Fan 2" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="hue:0210:00178824e7e9:14:color" }
Color FF_Light_FamilyCF_3 "Ceiling Fan 3" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="hue:0210:00178824e7e9:15:color" }
Color FF_Light_FamilyCF_4 "Ceiling Fan 4" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="hue:0210:00178824e7e9:16:color" }
Color FF_Light_FamilyLeft "Family Left" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="hue:0210:00178824e7e9:10:color" }
Color FF_Light_FamilyRight "Family Right" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="hue:0210:00178824e7e9:11:color" }
Color FF_Lightify_LED_TV "TV Backlighting" <colorwheel> (FF_Family, Family_Lights, Lights, gdashboard) ["iss:room:FamilyRoom", "iss:type:DevRGBLight"] { channel="osramlightify:rgbw:84-18-26-00-00-04-EA-17:color"}
And then in a sitemap, you can turn them all On or Off with
Switch item=Family_Lights mappings=[OFF="All Off", ON="All On"]