Extension group is to group related extensions. By default, Sellacious extensions are already in an extension group sellacious. These extension groups help to find the files of the same extension. Like if you want to translate whole sellacious, you can easily do that by selecting the extension group in languages. You can create these groups from Sellacious backend > Settings > Global Configuration > Language.
Type the name of the group you want and press Enter. Type the same name to other extensions.
Go to Sellacious backend > Languages > Translations. In filters, select the group you just created and extensions will be filtered by extension group.
By default, sellacious extension group is present, so you need to translate the extensions included in this group to translate your sellacious site.
With this, you can also create a group for other extensions and translate them.