Trainers can be enough for easy, dry and mostly paved routes.
If the event is short, flat and on predictable surfaces, a comfortable pair of supportive trainers may be completely reasonable. The fact that people are meeting outdoors does not automatically turn the day into a technical hike.
The point is to match the shoe to the ground. If your normal footwear already gives you enough comfort and grip for the route, there is no prize for buying heavier boots.
Mud, loose ground and wet rock change the decision.
The Ladybower, Chee Dale and Peak District photos above show why generic 'walking outfit' advice can be useless. Mud and rock ask much more of a sole than a pavement does.
For those conditions, deeper grip, a secure fit and footwear designed to stay stable on uneven ground become much more valuable.
Waterproof footwear is useful when the route is likely to stay wet, not because waterproof sounds premium.
A waterproof membrane can help in wet grass, shallow mud and prolonged rain. It can also make footwear warmer and slower to dry from the inside if water gets in over the top.
Think about the conditions you expect to walk in regularly rather than treating waterproofing as an automatic upgrade.
Try fit properly before chasing reviews and star ratings.
Heel movement, toe room, width and how the shoe feels on slopes matter more than a product winning a generic best-buy list. If possible, try footwear with the socks you actually plan to walk in.
The options below are examples for regular wet and uneven walking, not a claim that one shoe fits everybody. Use them as a shortlist only after the route gives you a reason to buy.