If I want to attack and there is a spot with multiple things on it, do I get to attack them all?

This depends on the ability you're attacking with...

If it is an attack ability, like Ra'Ha's Range Attack [2] ability, then you will only pick one target on that spot and deal 2 damage to that target. 

But if you're doing an area attack ability, like Ra'Ha's Area Range Attack [1], then you end up dealing 1 damage to everything in her awareness. This will include each entity on each spot in her awareness.

Can I beckon multiple summons on the same spot?

Yes, you can...

The keyword that allows you to do this is the word "place". When something is being placed on a spot, it does not care if that spot is occupied or not. This also applies for cards like Teleport.