The best way for you to make platinum would be to find mobs that are either green or grey that drop decent plat per mob or high value vendor trash like gems. The reason you want grey to green mobs is because if you want to farm platinum Kills per Hour is going to be a huge factor in how much platinum you are able to obtain in a given session. For example, Plane of Fire has pretty good vendor trash, but the mobs are in the mid-60's which would probably mean you'd be able to kill around 1-200 in an hour, which may net you 5-10k; but if you would have went to a zone such as The Hole, where the mobs are high-40's to low-50's you'd be able to farm substantially more just because you would be able to kill so many more mobs, maybe 2-400ish depending on how geared you are etc.
With all that said however, you'd make much better platinum if you were a higher level. If all you are concerned about is having some plat for spells and stuff a much easier alternative may just be to sell a krono. Selling a single krono should be able to buy your entire group their spells up to 105 and still have a bit left over for gear or other items you may need as you grow in levels, and even having a single character at 105 would be able to out farm your entire group of mid 70s.
