How protect interest of everyone while investing in property together?

Your question is not clear, as to why would 8 people intend to invest in 5 properties? What legal set up do you expect to protect the interest of all? 

As a rule, a property can be purchased either in a single or joint name, so you can purchase the same in joint names but when it will come to the division of property in an equal share in case any of the share holders in the property want to exit, it will be a legal issue unless pre decided by any agreement the concerned parties have mutually entered into.

In case you are looking at this kind of an engagement purely for business purpose, we suggest that you form a company wherein all the investors can be share holders and the properties are purchased in the company's name.

You may thereby have an internal agreement for each of the properties purchased and the various terms and conditions with respect to the exit / profit sharing / etc. can be drafted and entered into by the partners investing in that respective property.