Please see my responses to your questions:
VAT GROUP ON THE INVOICES OF VARIOUS COMPANIES
The invoicing should not be affected by the VAT group registration. The only change on the invoices is that all the group members should quote the group VAT registration number.
IS VAT EVER DUE ON SUPPLIES BETWEEN GROUP MEMBERS?
In most cases supplies of goods or services made between members of the same VAT group are disregarded for VAT purposes.
HOW IS INPUT TAX RECOVERY TREATED WITHIN VAT GROUPS? WHAT HAPPENS IF THE GROUP IS PARTLY EXEMPT?
Input tax recovery is determined in accordance with the use of the group, as a whole, of the goods and services received by each individual member. As a group is treated as a single taxable person most of the partial exemption rules apply to a VAT group in the same way as they would to a stand-alone business.
Input tax incurred by all the group members can be deducted to the extent that it is attributable to supplies made to persons outside the group which carry the right to deduct input tax.
WHO IS LIABLE FOR THE DEBTS, OBLIGATIONS AND VAT ERRORS OF THE VAT GROUP?
Since a VAT group is treated as a single taxable person, the limit for voluntary disclosures of errors on past VAT returns applies to the group as a whole. If you fail to make a voluntary disclosure of misdeclarations you may render the group liable to a misdeclaration penalty as well as interest. The group is jointly and severally liable.