Account sharing is a violation of the EULA as stated in our EULA section 2 B:
"You will be asked to select a password during the registration process ("Password"). You may not disclose your Password to anyone or allow anyone to use your Password to access the System or play EVE. You are responsible for maintaining the confidentiality of your Password and for any damage, harm, lost or deleted characters, etc. resulting from your disclosure, or allowing the disclosure, of any Password, or from use by any person of your Password. You may not obtain, attempt to obtain, use or attempt to use the password of anyone else. You are responsible for remembering your Account information and Password.
You will be assigned a login name and a character name during the registration and character creation process. You may not allow anyone to use your login name or character name to access the System or play EVE. No player may use the character name of another player to impersonate or falsely represent his or her identity. You may not obtain, attempt to obtain, use or attempt to use the login name or character name of anyone else.
You are encouraged to use a pseudonym, but you may not choose a name that violates anyone's trademarks, publicity rights or other rights. Acceptance of a pseudonym by the System does not mean that your chosen pseudonym does not violate anyone's rights. You are responsible for any liability incurred by yourself, CCP or anyone else due to the pseudonym you choose. CCP retains the right to reject any pseudonym it determines, in its sole discretion, is unlawful, indecent, obscene or otherwise violates standards of good taste."
It is however possible to move individual characters between accounts via the character transfer option in the account management pages. This is the only legal way to share your character with other players.
If you share your account with another player, you risk damage being done to the account and also it can lead to warnings or bans issued to the account.