First things first is ensuring that each member of your group has a valid passport for travel and making sure you allow enough time for any new passport applications to be processed.
Some countries require that your passport be valid for up to 6 months beyond your return date so don’t forget to check expiry dates and get any renewals done in plenty of time before the trip.
This link gives you up to date details on entry requirements and passport validity information for each country along with travel advice.
The EU has indicated that visas will not be required for short-term visits, but passport requirements will change a little. Check out the details on the Gov.uk website here.
You should find answers to any questions about getting or renewing a UK Passport here.
Collective (Group) Passports
If your group are all British nationals and under 18 years old on the return date, it may be possible to apply for a collective passport for the whole group! They cost £39 and you should allow 6 weeks for the process. Read more here.