You can use Settings --> Accounts and Imports --> Grant Access to your account... but if you want to add many users, seems tedious as for normal account, it only allows to add one account at a time.

Since, you have business Google Apps account, you can take their support service.

The approach you are using the standard approach where group is used that I feel is best approach. However, alternatively, you can use Settings --> Accounts and Imports --> Grant Access to your account... but if you want to add many users, seems tedious as for normal account, it only allows to add one account at a time.

Since, you have business Google Apps account, you can take their support service.