Pi and Pe aren't functions. Sometimes you'll see this used to describe the introverted and extroverted versions of the Perceiving functions. For example, Pi can mean either Si or Ni while Pe can mean either Se or Ne.
This is my first time ever seeing anyone use Pe and Pi at all. amnorvend is right on the money here.
To figure this out, you'll need to break down the dominant, secondary, auxiliary and inferior (shadow) functions. If you're an introvert and a P, this starts with the rational function (T/F) and if you're an extrovert, P begins with the irrational functions (S/E).
Lets take INTP and ESFJ.
The primary function of the INTP is Ti. Introversion + Rational.
The primary function of the ESFP is Se. Extroversion + Irrational.
The secondary function of the INTP is Ne. Inverse of orientation + Irrational.
The secondary function of the ESFP is Fi. Inverse of orientation + Rational.
The auxiliary function of the INTP is Si. Back to the primary orientation (introversion) + opposite of irrational.
The auxiliary function of the ESFP is Te. Back to the primary orientation (extroversion) + opposite of rational function.
The shadow of the INTP is the exact opposite of the primary function. Fe.
The shadow of the ESFP is the exact opposite of the primary function. Ni.
Now, all you need to do is read through this about a dozen times and memorize how it works. You'll start to see the patterns in no time.
So, an INTP is Ti, Ne, Si, Fe
An ESFP is Se, Fi, Te, Ni
An INTJ is Ni, Te, Fi, Se
An ENTJ is Te, Ni, Se, Fi
You'll see that Ps are extroverted in their irrational function as they conform themselves and their thoughts to the massive sea of information about them as they see fit. Js are introverted in their irrational function, which means that they stay fixed and pick and choose what information they will allow themselves to examine. If you think of the abstract, wishy-washy perceptions of the P and the staunch stability of the J in this way, it may make some more sense.
Ji and Je are also used similarly. I haven't heard it used in a long time, so no guarantee of being correct on this, but I believe to be "Pe"-dom, the dominant function is introverted Sensing or INtuition (perceiving functions), as Am said, and to be "Je"-dom is to be dominant in Ti or Fi (judging functions). "Pi"-dom is then Ne and Se dominance, and "Ji"-dom is Te and Fe.