"I'm planning on installing some agents manually on existing systems, and I want to have them put in a specific asset group once they're discovered. I don't see any way to tell the install script to put the asset in a group. How can I add the assets to a group, either through the UI or the CLI?"
There are a few ways.
attach -n| --gear <asset name> -g| --group <group>
You can also use -U| --uuid <UUID> to specify the asset if you have multiple assets with the same name.
In the UI, you have a couple of options. You can select an asset and click Add Asset to Group to add it to a group you select.
Alternatively, if you're trying to make a group for assets with a specific characteristic, you can specify rules that will automatically add assets to a group based on that characteristic.