Hello,
GST liability would arise if the maintenance subscription is more than ₹7,500 per member and the annual turnover of the society by way of supplying services and goods is ₹20 lakh or more. Since your society's maintenance is below Rs. 7500 limit, GST is not applicable.
Investment of funds would depend upon the duration for which funds need to be parked as an investment. For the maintenance of the premises on a regular basis, for payments to its employees and workers, costs of electricity and water, etc, funs can be parked in Liquid Funds could be accessed on notice of a couple of working day. While for meeting expenses which are not regular but usually needed after a few years like renovation, funds can be invested in FDs or fixed maturity plans (FMPs) from mutual funds with a higher return rate.
I hope this answer satisfies your requirement.
Regards,
CA Hunny Badlani