On the professionalism worry: a group is not a billboard, it is a room full of people asking questions. If someone from the brand answers those questions helpfully under a real name, that reads as more professional than a polished ad, not less. On the hidden profile idea, I would be careful. Admins of active niche groups watch for exactly that pattern, and a fake profile caught pitching your product does more damage to the brand than posting openly ever would. Since you have not worked in groups before: pick 3-4 groups where your customers already ask questions, spend a week just reading what gets posted and what gets removed, then start answering. After a few weeks of that you will know whether it earns its place next to the ads budget.