Use "of which" or "of whom" to combine two sentences like this:

I play golf with three guys. Two of them are doctors.

I play golf with three guys, two of whom are doctors.

The first sentence talks about a number of people or things in a group. The second describes members in that group.

We came across several snakes on our hike, one of which was poisonous.

There were hundreds of questions to answer, none of which made any sense.

He introduced me to a bunch of his friends, all of whom were really successful lawyers and business executives.