Best way to find groups to promote your launch?

Paid lists here on PH if you're in a hurry to promote

Linkedin and facebook groups, even X sometimes

There’s a lot of Website Directories you can list your Site on.

This depends on your product. I suggest to first identify your target audience by conducting market segmentation ( Demographic, psychographic, geographic, and behavioral ) then, look which social media platforms your audience uses and create a community engagement strategy

Just search on LinkedIn