Info on Informix userid's

Hello, I have been working on Oracle for a couple of months. Recently, I got into Informix. I have been trying to figure out how to create user id's on Informix. The manuals have not mentioned anything about this. From whatever I have read so far, I got the impression that anyone who logs onto unix can access Informix. Is it correct? How does Informix take care of security of data? Thanks in advance, Julie Joseph Infosys Technologies Ltd Bangalore India.