Talking about co-authoring is a bit like opening Pandora’s box. Approaches differ largely across universities and countries. Yet there are actually international agreements determining who should be named as an author vs. who should be thanked in acknowledgements (only). Even the order of authors has been regulated. Here’s a link to the “Vancouver protocol” on the matter, and the text in brief:
“The Vancouver Protocols state that in order to be credited as an author, each and every author on a publication needs to have been involved in the:
1. Conception and design, or analysis and interpretation of data
AND
2. Drafting the article or revising it critically for important intellectual content
AND
3. Final approval of the version to be published.”
Note all the AND-s in the requirements.
So why bother? Statistics on the matter show an increase in domestic and international co-authoring, thus the topic should be of rising interest.

Trends in the co-operation in science, 1985-2007
For this and more see the OECD Science, Technology and Industry Scoreboard 2009.
Gyöngyi