"Internal" license for group software?
Software developed by multiple parties under university funding is implicitly covered by the copyright of the university.  The attitude towards intellectual property ownership varies by university, so you should search to find out your situation.  

A few universities are very liberal about intellectual property.  See https://uwaterloo.ca/research/waterloo-commercialization-office-watco/intellectual-property .  Other universities are more traditional, see http://www.research.utoronto.ca/faculty-and-staff/inventions-commercialization-and-entrepreneurship/invention-disclosure/ .  An invention leads towards a patent.  Software development (and publication) lead towards a copyright.  

From this point on, you might have to consult a lawyer, but here's some intuitions that may help (but could be wrong).  

Practically, if the source code has been published in a book, the copyright may have been transferred from the university to the publisher.  If the code was rewritten and then put under an open source license, with attribution to the original sources, this could be considered a new work.  With copyright, the analogy is relatively clear in the academic world.  It's okay to cite the source, or use snippets as "fair use", but copying the whole text outright would be plagiarism.  

If the university is not interested in commercializing the software, there may already be a process in place to have it reassigned under an open source license.  The MIT license may be the simplest to implement.  The Apache license and LGPL 2.1 licenses are also popular.