Group VAT registration
Two or more companies or limited liability partnerships - known as ‘bodies corporate’ - can register as a single taxable person or VAT group if:

each body has its principal or registered office in the UK
they are under common control - for example, one or more company is a subsidiary of a parent company

PWC's explanation:

Group taxation for VAT
The VAT Act allows all companies that have established business presences in Hungary and qualify as related enterprises to form a VAT group. The essence of a VAT group is that its members act under a single VAT number in their transactions (i.e. they issue invoices under a shared VAT number and submit a single, joint tax return) and the supplies of products and services between the members do not qualify as business transactions from a VAT perspective.

Link: http://taxsummaries.pwc.com/ID/Hungary-Corporate-Other-taxes