Common practice when implementing new computer systems is to train the minimum required with the intention of undertaking more training later. Unfortunately more often than not, does not happen.
What you can do
Step 1
Look at job roles and determine the system functionality required. Then ask or assess how well the functionality is used. If it is not used well undertake training to improve it.
Step 2
Ask people if there is other functionality within the system which they would like to know more about. You may need to explain what functionality there is. Prioritise the list and agree the priority with them. Then arrange for someone who knows how to use the functionality to train them, working your way down the list.
The training resulting from the above steps should should be done either on a 1-1 basis or in small groups of 2 or 3, anymore people and you will need a fully structured training session.
Even if you believe the requested functionality is outside a person's job role considering training them on it, as long as there are no security issues. The more comfortable people are with the system the better users they will be.