How Musk can get 100 colonists to Mars every two years

Actually, it all depends upon how many launches he can do.  He doesn't necessarily need a really big rocket either.  But he will probably need something bigger than a Falcon Heavy.  A Saturn V class rocket could do it.  Maybe.

It may be possible to send 10 at a time, but launch 10 times during a launch window.  A launch window could last as long as a month.  Since he may be able to set aside at least two launch pads, that means five launches per pad.  If he gets a third operational, that means just three or four.

Getting 10 at a time is a lot easier than getting a 100.

Use of launch windows could do that, but what about the earlier discussion?  There will be two years between launch windows.  That means a lot of time could be devoted toward massing methane/LOX at a refueling station at EML-2.  Most of the LOX could come from the moon.

He would need a base on the moon to mine the LOX and to launch it to EML-2.  He may want to reuse the shuttle craft any number of times.  Add the refueling base in order to complete the facility/system.

Delta-V for a Mars transfer orbit from EML-2 is less than 1 km/sec.  A rocket from Earth could refuel, then head on to Mars.  This would be repeated 10 times with 10 people on board on each trip from Earth to EML-2.  Ten launches times ten people per launch equals 100 people per launch window every 2 years.

Each launch from Earth could put 100k pounds or more at EML-2, assuming that a Saturn V class rocket delivers it.  Let's say you bring 20k pounds of methane from Earth.  That means that 80k pounds of lunar oxygen from the lunar base could give you as much reaction mass to land on Mars as landing on the moon.  That 100k can be the reaction mass from EML-2 to a Mars landing.  You may not need that much.  Or you may need more.  I haven't checked that out as of this writing, but it is possible to use aerodynamic braking in the Martian atmosphere to get a lot of that Delta-V without using fuel.

Another possibility is to make heat shields on the moon.  You have two years to prepare for each launch window.  This means that you have less to launch from the Earth, which would be helpful.  If you were really ambitious, you could make fuel tanks on the moon, too.

This leaves 80k pounds that can be used for the life support hardware and fuel tanks from Earth.  He'd have to cram 10 people in there, and they would have to be able to survive such a journey of 7 months, then land on Mars.

Even if he could do all this flawlessly, he isn't finished yet.

He may want a base prepared on Mars before landing there.  You've got a 100 people coming in, and they may want to stretch their legs.  That provided that they could still walk.