Billionbrains Garage Ventures Ltd., the parent of India’s largest investment platform Groww, is preparing to file for an initial public offering in Mumbai as early as next week, according to people familiar with the matter.Backed by Microsoft Corp. Chief Executive Officer Satya Nadella, the firm is seeking to raise between $650 million and $800 million at a valuation of up to $8 billion, the people said, asking not to be identified as the information is private....