This is the worst advice, never spam a group, group Admin's hate that schit as do users. Post judiciously to groups, ie depending on how busy the group is that may mean once every 2 weeks or 2 months. Run genuine promotions, now that's where the best brand promotion lies, everyone loves a deal so find a great loss leader. Many companies have realised it's either spend on advertising or use that same money on a loss leader, the latter actually gets guaranteed sales and people onto your website, if you back the sale up with a great product and good service you'll win a customer to sing you ur praises far and wide.
And one of the best is know your product like a boss and give advice including your products as solutions, but don't be a shill. I've seen a camera store work like that, nobody minds and the guy drives traffic/sales virtually daily.
It's a fine line and it takes work, no free lunch.