Hey Lifesci
The main thing you need to know is the forward voltage of each led - add the voltages up if you are running them in a string. The forward voltage can be found in the led data sheets. Your driver needs to be able to supply at least that voltage plus a bit more - it gets a bit involved but be sure your driver can supply at least 10% more current than you need to drive the leds at their maximum.
The voltage the driver supplies is not critical as long as it exceeds the total forward voltage of your led string. As you noted, the drivers job is to regulate current.
You can only add another led (I am assuming you mean in series) if the additional added voltage does not exceed the drivers limit. If you are running the leds parallel, then adding an led doesn't affect your voltage - however, the current is split between parallel strings. So if you are supplying 1 amp to one led (or series of leds) adding another led (or string) in parallel will half the current each string sees to 500 mA (without affecting the forward voltage).
If you are running parallel leds (or strings) off one driver it is critical that the total forward voltages are the same in each string.
Hope that is not too confusing!
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