Hallo, all 
I'm trying to figure out the easiest way to drive 21 groups of LEDs. The groups are comprised of 2 to 7 LEDs. Probably only four or five will be active at one time.
My first thought was to make a 3x7 matrix, which was easy enough to get working on my diecimila, using only 21 LEDs: sourced three, sinked seven; only ever had 1x7 illuminated at a time, to avoid pushing too much current through the pins. But I know I can't put more than one LED in each part of the matrix, so this particular setup won't work.
On the playground, there's the schematic of using a transistor to drive more LEDs, but I'm not sure if I could convert that to a matrix. Connecting the emitter of the transistor to an arduino pin would still be pushing too many amps through the sinked pin, wouldn't it?
Would adding transistors onto both sides of the array work?
Is this even a viable solution, because I might need to cycle through the columns/rows faster than the transistors can manage (I'm not sure this would be such an issue)? Perhaps a chip like the Max7221 would be a suitable replacement for driving the matrix?
thanks in advance
Look at my monome project:- Econo Monome
This has two LEDs per place but you could connect as many as you want in parallel. You have to get extra current capability by both sourcing and sinking current with external components or chips. In this project I used a FET to source the current and a chip containing Darlington drivers (transistor pairs) to sink the current. Note each LED needs its own current limiting resistor, and you need software to multiplex the display.
You can do this with LEDs in series but you need to supply a higher voltage to the source FETs and have a transistor to get the gate signal up to this level.
Thanks for the tips, Grumpy
I'm up for the challenge of programming the multiplexing...I understand that concept. It's the hardware side of things that I get lost on 
I just get the feeling that there might be an easier way, both for the hardware and the software. Of course, I am no expert in either of those, and could just be jumping into the deep end without knowing it 
Most design is a compromise, like a water filled balloon. Squeeze one part and another bulges out.
Yes there are ways of controlling LEDs that are easy on the software but heavy on the hardware (separate addressed latch for each LED). Then again you can get light on the hardware but heavy on the software. (charliplexing). So a matrix sits somewhere in the middle.
True enough.
I'll keep an eye on the econo-monome, and work myself down from there.
Thanks!