What is the minimum CGPA that is required to sit for all the placement interviews?

There is no set minimum CGPA requirement for placements. Each company has it’s own criteria.
However, a trend is being observed. Every year, companies are setting higher and higher CGPA as the qualifying criterion. This is mainly because of the ever increasing number of students. Companies usually have a set number of candidates they expect and the cut off lists are generated keeping this in mind. Say a company wishes to consider x number of candidates, the top x would be allowed to appear for the exam and this is how the cut off is decided. This year (2014 graduating batch), cut offs went as high as 8.95 for companies falling under the super dream category (CTC above 10 l.p.a.) while a few companies kept the cut off as low as 6 too.
I would recommend keeping your CGPA somewhere close to 9 if you wish to be eligible for most, if not all companies.