Convention to denote author who left the research group? We at a company would like to publish a whitepaper to be included in the proceedings of an academic conference. The specified format requires us to list all authors together with their e-mail addresses.

One author (who did about 5% of the work) had parted ways with the company about half a year ago and had no connection with the ongoing research (or members of the research group) at all since. Thus he doesn't possess a company e-mail address any more either.

How should such a person listed among the authors? He contributed to the research, so naturally should be listed but I'd prefer to do it in a way that communicates "I have no connection with the group any more, I don't know about recent developments and if you contact me most possibly I won't be able to tell you too much useful information". Also, is it suitable to include his personal e-mail address?
From your question, I am concerned that you have not talked to the "author" about the manuscript. You must get permission from all authors prior to submitting a manuscript. It is not acceptable to submit a manuscript without permission from all authors nor is it acceptable to leave an author off just because they have left the group.

As for what is acceptable for an email address and affiliation, almost anything goes (as long as it is not misleading). I personally think universities and research groups should give permanent forwarding addresses to eliminate/reduce these issues.

In terms of the affliation, you should also ask the author. It is not uncommon to list an affiliation as the company/university where the work was done and provide a new corresponding address. As for email, it is common to list only the current address. This might be a new corporate email address, assuming it is appropriate. If not, a personal email or a new personal/professional email address is fine. Again, just ask the author and do what they want.