Player Spawns And Team-AI Idling

For players and Team-AI to spawn, you need to set up PlayerSpawner Elements. By default a new project already has a PlayerSpawner Element, however you should create at least 3 more so players and AI don't clip into each other when spawning.
To set up Player spawns, spawn at least 4 Player Spawner Elements at the positions you want the heist to start. To keep things organized, you can place a MissionScript Element and let that one execute those 4 player spawns. Player Spawner need to be executed on startup, or by the startup-element.
You can place as many spawns as you want. The game will automatically spawn players and Team-AI randomly at any executed PlayerSpawner element.
A common setup is to have multiple groups of PlayerSpawner elements, with separate MissionScripts executing them. Instead of the startup element executing all PlayerSpawner at once, it executes a Random Element, which randomly picks one of the spawner groups.

To add idle animations for AI-Teammates, place an AreaTrigger over the same location as a player spawner and set it the instigator to ai_teammates. The area trigger then executes a SpecialObjective with the animation you want. Make sure to disable the area triggers after spawning, otherwise they may accidentally trigger again mid-heist if an AI-Teammate walks through them.