My friends and I will have a trip for Kenya and Tanzania in the end of Sep., basically go Safari and climb Kili.
I checked some website of tour operators, and found the price is variable according to the member numbers.
Does that mean I should find members in advance by myself? Or the tour operator could help us find more?
A good tour operator will find you a group to travel with as long as you specify that you wish to travel in a group. You do not have to worry about finding a group yourself.
Normally a tour operator can find you a group to join and make prices cheaper, although it might be easier if you have other friends who might want to join in, if not then its also possible to do a trip just the two of you in a reasonable price. One thing i learned being a native of Tanzania and Mountain guide for 13 years is ultimate experience you might get by being with the right organizer and right staff both on the Mountain and Safari.
The important thing is to make sure you receive enough information about the trip.
How many days are you planning to spend on Kilimanjaro and Safari, and which route are you planning to use, from my experience Machame route is the most scenic route and you will enjoy it. If you need any specific information let me know.