First of all - You are working with a really old version of the product, so it is possible that things I say here don't comply with your version.
There are three layers of security:
CONNECT, RESOURCE and DBA
The privileges can be granted to a user if you don't want to specify access per table or column.
You can also create ROLES and grant these roles to a user.
Most of this stuff can be found in the sql syntax guide. Check out the GRANT statement. All manuals are available on the web.
http://www.ibm.com/informix - library - documentation.