The essentials
Comfortable running shoes and clothes, water, a charged phone and your return-travel plan are a useful starting point. Carry any personal items you normally need. Confirm whether the group has a secure place for bags; do not assume it does.
After dark
Check route lighting and visibility. Consider a light where needed and clothing that other road users can see. Make sure you understand crossings and regroup points before the group sets off.
For an organised event
Read the compulsory-kit list. Keep your entry confirmation and bib-collection details accessible. Ask about storage, toilets and water availability instead of assuming every race offers the same facilities.
Keep it familiar
A social run is a good time to learn what works for you. Borrowing advice is useful, but your comfort and the organiser’s instructions matter more than buying everything someone else uses.