"To take something on" means to assume responsability or to accept a challenge.
"To take somebody on" can mean "to employ" or "to engage" in a job. For example "She was out of work for several weeks, then a local shop took her on as a salesgirl."
However, here it is used in the sense of being prepared to fight the other members of the gang. He is saying that he is prepared to fight them all at the same time if they object to him not joining the gang.