How can I apply the ISO standard within a group of companies managed by the same Quality Management Center?

The unified Quality Management Center has to break down the work flow of the group in order to work on each company by itself.
The ISO standard is a standard after-all, and you would have to conform each company's processes, forms, workflow, etc...
to that standard.

Hi, you have to nominate first a deputy management representative for every company, identify the scope that which companies you want to implement the ISO in it then implement the standard in each company aside tailored according to the company needs and respecting the medatory process to establish as per the ISO standards then apply the continual improvement concept(deming wheel -PDCA)