Most people think that Rengar is a weak champion that can't do anything in teamfights, so that gives you an advantage.
They won't expect Rengar to do good damage until late game comes, then people will notice a little bit that you can do more than they expect.
In teamfights what you should do is ult and go for the ADC, if you manage to kill the ADC you should go for the person that is killing your ADC, make sure your ADC gets a good amount of kills, you won't be able to carry a game without your team.
This guide is mostly about you destroying their team.
If you manage to earn all the items+a fully stacked necklace you won't be able to NOT take kills.
And I will warn you right now about 1 more thing, the people that are hard to gank are people with escape/initiation abilities.
For example: Kassadin, Fizz and Ahri, these are champions that can avoid getting ganked, but if you can manage to maybe damage them if you know you can't kill them that will be good for your laner if he/she is getting destroyed.
So this is the small tips I can give you for now, have a fun time killing your enemies!