Having only been a spectator watching from my beach chair on the grassy hills surrounding many Cape League ballparks over some 40 summers my understanding of the inner workings (ie. how one gets on a roster) is limited. However, my dog "Scout" (yes, dogs can watch too) and I have arrived at a few conclusions:
1. Being a rising college soph or junior MLB projectable STUD helps
2. Being from one of the Power Conference schools who supply said STUDS also helps. You will notice many D-1 programs send guys to the same Cape team every summer. Usually a college coach knows and trusts a particular Cape League coach, say a PC, who the college coach can send his player to for well managed summer reps and exposure. Paramount to the college coach doing the sending is being able to trust the Cape coach, whom he is entrusting his "Friday night guy" with for summer, to come back to campus in the fall healthy and having learned a thing or two from the night in and night out grind of a summer of ball on the Cape. So if a college coach has a good experience with the coaches in say Cotuit, or Orleans, or Chatham, he will most likely send his players again to same team next summer. This is not written in stone but there are tendencies and, as TPM alludes, you can see these tendencies by looking at the summer rosters.
3. There is a bit of a "wild card" factor to the Cape League however. I have seen many players over the years who are NOT from the Power Conferences and many from D-2 and D-3 (usually pitchers) who have made it to "the last best league." Carter Capps (RHP, Miami Marlins) played for D-2 Mount Olive and had a breakout summer on the Cape and I remember an LHP from D-3 Williams College, Harry Marino who put on the Chatham pinstripes in the summer of 2011. Marino later signed with the Diamondbacks as a free agent out of D-3 Williams.
4. There's even a "back door" for Cape League coaches to usher in guys who don't fit the normal STUD profile and this gets used often, usually in the first few weeks of the Cape League season. For example, the CWS does not conclude until late June. There are several teams competing in Omaha and regionals every year who obviously have guys scheduled to play on the Cape that summer. Well, the Cape League season begins around June 10 every summer with games nearly every night. So a Cape League coach who had his roster all set (rosters are usually set by end of fall ball for the NEXT summer) suddenly come June 10 is missing 3-7 guys? Guys who are still playing in CWS or even guys who got injured and suddenly are "no shows" as of opening day???
Cape coach now gets on phone and beats the bushes to find the best available talent, that can get to the Cape as soon as possible. Like, "Can you be here tonight?" kind of situation. In this scenario it certainly helps to be from east of the Mississippi when said call is received, and preferably in New England or Long Island at worst. Often these "temp jobs" are for only a few days and terms are agreed to with a temporary contract not the full summer contract the STUDS get.
But for some it is the chance of a life time to play in a league, whose pedigree they do not fit, even if for only 8-days. And they can forever say, "I played on the Cape in college." And anyone who knows college baseball will understand what that means.