SoundSoap is REALLY straightforward, but probably won't be great for this situation. SoundSoap Pro has 4 tools with different approaches to get rid of the sound; the good news is, it isn't that hard to figure out (somewhere, maybe Bias' website, I found a tutorial video when I first started using it), the bad news is the price, maybe more than you'd want to spend unless you plan on doing this a lot. I think Bias lets you install the software for 14 days before you need to authorize it, so that may work for you to do some fixing now and then if you really like it get it to use down the road. Keep in mind, SoundSoap Pro is a plugin, so you will need to run it within some other AU or VST host--I am assuming Garage Band can handle this? I have never used Garage Band and deleted it from both my machines so I'm just guessing but it would seem silly if it couldn't support AUs.
About the EQ, the MasterWorks EQ plugin is something that comes with Digital Performer (my audio/MIDI sequencer of choice) and it probably isn't worth the money to buy a sequencer just for an EQ plugin. I only brought it up because it is my favorite (and maybe you have a friend with Digital Performer, maybe not) because it lets you see a representation of the sound so you can here and see what you did.
While it may only make a small difference, the EQ idea could work with any EQ plugin in any software. Again, if you have Garage Band I'm guessing it has Apple's super simple EQ AudioUnit. If you have access to Garage Band and this EQ, then just very carefuly adjust the sliders up and down a little at a time until you think you are making a difference, then go back and listen to the original and compare--you may have convinced yourself the EQed version sounds good, but your ears may be playing tricks so comparing to the original will let you know if you destroyed the band while getting rid of the crowd. Keep in mind, if crowd noise means people's voices, that will be the same frequency as a singer making removing the noise that much more challenging with only an EQ and not something like SoundSoap Pro.