TL;DR Get a VITUR Visa which costs Rs. 1600 and is valid for a maximum of 5 years. Write a letter to them explaining your situation and you'll be good to go!
A list of documents and important information is also given on the New Delhi Embassy website.
As you might have already guessed you're gonna need a Tourist Visa of the VITUR Category. The price of this Visa is Rs. 1600 (Because you're an Indian Citizen). I'm going to go ahead and assume that you and your friends will be applying for the visa in India so it's payable in Rupees.
The visa is usually multiple entry and each entry allows you a maximum of 90 days up to a total of 180 days in a single year. The visa may be valid for a maximum of up to 5 years but this is entirely up to the discretion of the embassy issuing the visa. The form on VisaHQ Passport services which can be used to apply for the visa clearly states up to 1825 days.
You can write a letter to the embassy explaining why you would like a long term tourist visa. Also, I believe in some cases the embassy decides what kind of visa to give you. The New Delhi site does not mention this but the New York site does. An in-depth but old example of the same is given on the Consulate General of Chicago website.
Wikipedia as well as the Other Visas Section of the New Delhi site will also tell you that the visa is valid from the first entry into the country until the expiry of the visa.