I would assume that by sharing an account you are accepting the responsibility of everyone that logs into that account and any alternate accounts that you have or anyone that has accessed that account would then be considered ban evading. Otherwise it would just be too difficult to prove who the person was posting at the time of the ban.
mprep answered my question in another thread:
How are ban evasions handled with shared accounts. In a regular ban scenario, a user is banned (as a person) and thus all their accounts are banned.
If an account with shared access from users X gets banned, are all users affected by this ban, or is just the user that caused the ban (thus violated forum rules) banned together with their shared account.
If so, how is it determined which user is affected by/included in the ban.
AFAIK it's on a case by case basis. In this case, the primary owner of the account is apparent so he's the one responsible.