I'm making a platformer and I want to make it so if I wanted to have a new object kill the player, I would just add it to a group or something like that.
I'm used to programs like GDevelop and GMS2 and they have grouping (GDevelop has regular grouping and GMS2 has parenting)
I can't seem to find any documentation on this, any help?
Yes, object groups are a thing in Fusion. They're called qualifiers and can be found in the "Events" section of the properties panel. You just need to click on the blank area next to the "Qualifier(s)" section and an Edit button will appear. Upon clicking this you can select "Add" from the box that pops up and select a qualifier from the "Add a qualifier to an object" menu (it doesn't matter which one you choose). After you choose one you can then create events which target the qualifier group (it'll show up in the "New Condition" menu with icon of whatever qualifier you chose).