Four of us to use bayernticket for a Monday evening end of May to travel from Munich to Salzburg. Do we need reservations? Jack

No reservations needed.

Thank you, suggestions regarding how early to come. Are there seats? Do they allow everyone who comes on stay?

I usually get to the track about 10 minutes before departure time. The trains are usually crowded in 2nd class especially the first departures after the 9:00 AM allowed on the Bayern ticket. I actually paid more on my return trip from Salzburg and went 1st class on EC. It cost about 30€. I was solo. But with 4 traveling, it would cost considerably more for you. It was just really hot and I thought it was worth the extra 10€ for me.

Reservations are impossible on regional trains. You hop on and look for seats. Those who don't find seats have to stand.
Since the train to Salzburg begins in Munich it will be at the platform about 10-15 minutes before departure. Munich is a dead-end station. The cars in the back will be the most crowded. Walk ahead on the platform and board a car in the front half of the train.

BE THERE (on the platform) when the train is backed into the station...get on immediately.

I would not expect too much of a crowd on a Monday evening.
Get there a bit early, no need to make a big fuss out of it.
Depending on where you will have to go in Munich, you can already change to Munich's S-Bahn and subway system at Munich East - which is more convenient than at Munich central station because the first is not a terminus.

I am told the regional trains have no room for luggage. If the four of us bring luggage. Where to put it??

A regional train is like a city bus. You can sit down and put your luggage wherever there is space. You'll have some racks over the head where you can put small suitcases. If you have large luggage, you need to find some space somewhere.

Normally there is space close to the doors and I would probably just leave large luggage there and sit down somewhere else. But I can understand if you don't want to do it since you might not be able to keep and eye on your luggage.

In many regional trains there are spaces for taking bikes. They have folding seats and you can sit down there with a lot of luggage close to you.

But don't worry too much, with a little common sense you'll find a way to take your lugagge. You should be polite but not too polite. It won't endear you to your fellow travellers in a completely full train if you block 8 seats with your luggage. On the other hand, luggage is meant to be taken on the train, so don't bother to to try make it disappear so that nobody is disturbed.