Any suggestions please for six 19year old guys heading to Croatia. Flying in and out of Split, what are the islands off there that are good fun and have cheap rooms or camping grounds. They need to spend about 3 weeks all up in either Aug or Sept. And roughly the cost of cheap rooms??? Is it like the Greek Islands???

There are several party islands, but Pag, Hvar and Brač are without any doubt the most popular and visited. Here is a good presentation of the Croatian party beaches and locations:
http://www.croatia-beaches.com/party/best_party_beaches_croatia.htm
Now, I am about to suggest two places for them. One is definitely Zrće (Pag island) beach with accommodations in nearby place Novalja. The other is Hvar with its wonderful town and vibrant party scene. All other mentioned places are also great, but not so lively. Zrće beach and Hvar town have literally every day parties with really great number of participants.
The difference between the two places is that Hvar is more urban and and Zrće is a beach that is almost totally dedicated to parties and nearby Novalja is a good place to find an accommodation and is also well stocked.
As far as the weather is concerned, both the places are great during the months of July and August, but Hvar as more southern location has better climate at the end of August and September compared to Zrće.
The prices in Novalja for six bed apartments are around 130 - 145 EUR/day. For example, this type:
http://www.novaljaapartments.com/novalja-apartments/accommodation-2531.html
http://www.novaljaapartments.com/novalja-apartments/accommodation-2891.html
The prices in Hvar for six bed apartment are around 100 - 150 EUR/day. Fore example this type:
http://www.apartmanihvar.org/hvar-apartmani/smjestaj-2602.html
http://www.apartmanihvar.org/hvar-apartmani/smjestaj-2497.html
Both, Hvar and Zrće are easily reachable from Split airport.
In case that you decide for Zrće, you just have to take a public bus to Trogir and from Trogir there is a direct daily bus connection at 17:30 with arrival in Novalja at 21:30 ( http://www.antoniotours.hr/en/croatia/island-pag/pag/bus-line-novalja-pag-zadar-sibenik-split-s7.htm ).
In case that you prefer Hvar, then just take a public bus to Split, disembark close to Split harbor and take the first catamaran for Hvar. Catamarans depart at: 11:30, 15:00 and 17:00.
For party vibe - it realistically has to be July or August with late June and early September a faded version thereof.

I also recommend them to go to Zrce on Pag Island. I think the party in Zrce can hardly be compared with any other place on the coast and it's ideal for young people. Here are some photos-http://www.novalja.com/photoalbum?lang=en