I want to touch on something that you have been placing in the comments on the other answers:
But isn't resistance a property of how difficult it is for electricity to flow through material?
No. Well, partly. But that is a very over simplistic way of looking at things.
A given material has a Resistivity associated with it. What this is a measure of is how much resistance per metre a material will present. Actually it is related to resistance by the equation:
Where \$R\$ is resistance, \$\rho\$ is resistivity, \$L\$ is the length of the piece of material, and \$A\$ is its cross-sectional area.
If you take a \$100 \Omega\$ resistor, it will be made of a material with a known resistivity, formed into a device where \$A\$ and \$L\$ are selected to get \$100 \Omega\$.
If you take two of these, and place them in parallel, you haven't changed the resistivity of the material, or its length, but what you have done is increased the cross-sectional area - doubled it to be precise. If you increase \$A\$ in the equation above, you can see that \$R\$ goes down.
Perhaps an analogy would help. Electricity is the flow of electrons, so lets make the analogy that it is like moving water. Bear with me.
Imagine taking a glass of water and putting a drinking straw in it. You suck on the straw and water moves through. Now if you suck harder, you get more water through. You are behaving like a voltage source. You are trying to move the water through the straw which is offering some resistance. You can imagine this resistance if you compare the case of using a straw, to simply taking a big gulp directly from the glass without the straw.
Now try to drink with two straws instead of one (in parallel with each other) - it gets easier doesn't it? You can drink more water with the same amount of effort. This is because the two straws in parallel have a larger cross sectional area to let water through.
Same with putting two resistors in parallel.
For completeness, what happens if you put two resistors in series?
You aren't putting them in parallel, so the area doesn't change - all the electricity has to flow through one resistor to get to the other as opposed to flowing through both at once.
But you have made the length longer. To get from one end of the series resistor pair, the electricity has to flow through the first resistor and then through the second (over simplification). You've doubled the length.
If the length gets longer, we can see from the equation above that if the length increases, so does the resistance.
As to your analogy of the bouncers at the club. Well, you can't fight both at the same time, you would have to take on one, then on the other (or alternating between, whatever). So essentially by putting a second bouncer at the door, you have doubled the length of the path through the bouncers into the club, hence you have made it harder.
It's difficult to actually make that analogy into one where the bouncers are in parallel. It would be almost like if there were now two doors but still only one bouncer - it's easier to get.