Group size does renting one large vehicle become more practical than multiple ca...

I'm organizing a trip for around 15–30 people.

What are the most important things to consider when choosing transportation for a large group, especially regarding luggage, scheduling, and comfort?

Are there common mistakes that first-time group organizers should avoid?

I organized a trip like this once (in New Zealand). In this case bus (including a driver) worked well and was cost efficient (as compared to car rentals, van rentals, or public transport) but that may be different in Alberta.

What are the most important things to consider when choosing transportation for a large group, especially regarding luggage, scheduling, and comfort?

This does require a fair bit of cat herding. The bus can't leave until everyone is on it and you're only as fast as your worst stragglers.

1. Make sure that EVERYONE has a cell phone working in region. I would make this a non-negotiable requirement.
2. Make sure that you have all numbers readily available on your phone. Making a WhatsApp (or equivalent) group for group communication is also a great way to keep everyone up to date
3. Learn the personalities. Most people can easily manage themselves but there are always a few characters that wander off, don't know how to keep track of time and need looking after and follow up. If necessary assign them a "guardian" on a rotating basis.
4. Make a "bus buddy" chain. Assign a bus buddy to each person. Before the bus leaves, every passenger looks for their bus buddy and yells if the buddy is not there. Make sure that the bus buddies are NOT from the same group (friends, family, etc) but mostly strangers. Make a chain. Alice looks for Bob, Bob looks for Charlene, Charlene looks for Dave etc until Zach looks for Alice.
5. Be VERY clear about expectations and schedule. "You must be back the bus at 5:00pm sharp". Don't use terms like "in an hour", "after dinner". Be short, be concise, be specific and repeat often.
6. Do NOT tell people "the bus leaves at 5:30pm". Tell them when they need to be on the bus. You can then select a suitable safety buffer that depends on a variety of factors: how reliable and adult is the group, how far can people wander off, what the consequence of a late departure, etc.
7. Have an emergency plan and make sure everyone agrees to it upfront and is prepared. "If you miss the bus, you must take an Uber or Taxi to the next destination". Make sure that people know how to get an Uber or Taxi.