I would suggest using ini files over text files, ini files are implemented internally as hash tables so their performance should be nearly the same (and much better than using $read(,nw)).

In this way you can have a single file with multiple sections and users.

activity.ini:
Code:
[#chan1]
nick1=1
nick2=1
nick3=1

[#chan2]
nick4=1
nick5=1
nick6=1


Then you use $readini(activity.ini,n,$chan,$nick) to access that information. Instead of storing just a "1" you could store other information like last join time to use for limiting when you welcome them.