Volcano climbing in indonesia

i am currently looking at various trekking companies to make a trip up mount
rinjani ideally or if need be perhaps another volcano in indonesia. does
anyone here know of a way to sign up other ppl that might be making the trek
there during the same time im going (oct06)? the reason i ask is most
agencies are almost 2x the rate for a single person which is understandable
but if i can avoid it with a partner it would be helpful.

also can anyone recommended an agency that they have tried trekking? i went
climbing mt. kilimanjaro last year and only after i got there did i realise
it would have been cheaper and easier to just buy a package locally instead
of getting a higher rate by booking abroad.

thanks