How can I get brighter photos at live concerts in low light?

It's all about the lenses, technique, and postprocessing.

Photography in a theatrical setting is one of the most challenging forms there is, both in terms of pushing the equipment you use to the absolute edge of their capabilities and in terms of requiring every bit of skill and experience you might have as the photographer.

EOS 5D Mark II + EF 24-70mm f/2.8 L, 70mm, ISO 1250, f/2.8, 1/640 sec

Photography is the art of capturing light. Most concert/theatrical settings don't offer much light to capture and what light there is to capture is changing rapidly and the subjects are usually very animated.

EOS 5D + EF 24-105mm f/4 L IS, 24mm, ISO 3200, f/4, 1/50 sec

The traditional solution to not much light (longer shutter speed using a tripod to hold the camera still) doesn't work because nobody on stage stands still for 10-15 seconds while you take a picture.

Manual exposure with Evaluative metering at about -1. EOS 5D Mark II + EF 70-200mm f/2.8 L IS II, 200mm, ISO 1600, f/2.8 (notice only the front line is in sharp focus), 1/200 second. I'm right on the edge of motion blur at 200mm and 1/200 second even when timing the kick at its apex. But the faces are sharp and that's what counts the most.

The traditional solution to capturing motion (faster shutter speeds) doesn't usually work because there often isn't enough light to capture a good image on a small sensor with a narrow aperture.

Under dim red and blue LED lights: Canon 5D Mark III + EF 50mm f/1.4, ISO 5000, f/2.8, 1/80 second and a LOT of post-processing.

In the end you have to balance the two as best you can AND use gear that allows you to capture as much of the scarce light that is present in the scene in as fast a time as possible. That means fast lenses (wide apertures), larger sensors, and cameras that are highly responsive (fast handling and AF). It also means you must use impeccable timing when you release the shutter. If a performer is jumping you can get by with a slower shutter time if you catch the performer in the instant they stop going up and start going back down. If they're running back and forth across the stage, catch them in the instant when they transition from moving right to moving left or vice versa.

EOS 5D Mark II + EF 70-200mm f/2.8 L IS II, 200mm, ISO 1600, f/3.2, 1/200 second. (I could have gotten a slightly better result at f/2.8 and 1/250 if I'd been totally on my game!)

The accepted answer to this question gives a general overview with more than a few links at the end to other questions here at Photography.SE that deal with individual aspects of concert/theatrical photography.

EOS 7D + EF 70-200mm f/2.8 L IS II, 200mm, ISO 3200, f/2.8, 1/1600 sec

You can also check out all of the questions here at Photography.SE with the [concert] and [theater] tags.