1. You should use the Informix account to manage the database, such as Grant/revoke permissions, creating/deleting tables, dbload, and so on.
2. Should you create a separate account to do these things? Why?
3. Should you use the Informix account to manage the database application and place all users in this group?
Instead of doing everything with the Informix account, you should have an Informix account and an Informix group, and the account number and group are unique. There should be only one user of Informix in the Informix group.
We have an "application" account, which is the owner of all the non-system tables, and uses it to set all permissions, with all the resources, data, and executable directories and code. The purpose of this is to isolate two different types of work, data management and server management. The functions of the DBA are independent (and only one person can do this), and if your database is large or about to become large, you will appreciate the benefits. If you break out of this principle, there will be serious security risks. Other users within the Informix group can perform many of the applications in the Informixdir directory, which you do not want to execute. So there's no need to risk putting all the users in the Informix group.