Copy contents of "upload" folder (extracted inside main downloaded grouped_addons.zip archive) to WHMCS root folder. Future update will require you to overwrite files. WHMCS Admin -> Setup -> Addon Modules...

File configureproduct.tpl. Replace condition from {if $addons} to closing {/if} with code below to support grouped addons. {if $addons} <style> .panel-grouped-addons...