New Twiddle

components.item-component\.js

import Component from '@glimmer/component';

export default class extends Component {
  get isActive() {
    return this.args.currentItem === this.args.name
  }
}

controllers.application\.js

import Controller from '@ember/controller';
import { tracked } from '@glimmer/tracking';
import { action } from '@ember/object';

export default class ApplicationController extends Controller {
	@tracked
  selectedItem = '';

	@tracked
	canSelect = false;

	@action setCanSelect(e) {
    this.canSelect = e.target.checked;
  }

	@action update(name) {
    if (!this.canSelect) { return };
    this.selectedItem = name;
  }
}

templates.application\.hbs

<input type="checkbox" {{on 'click' this.setCanSelect}}>

<GroupComponent @activeItem={{this.selectedItem}} @onSelect={{this.update}} as |g|>
	<g.item @name="foo"></g.item>
	<g.item @name="bar"></g.item>
	<g.item @name="baz"></g.item>
</GroupComponent>

templates.components.group-component\.hbs

<div style="border: 1px solid">
{{yield
	(hash 
    item=(component 'item-component' 
      currentItem=@activeItem
      onSelect=@onSelect
    )
  )
}}
</div>

templates.components.item-component\.hbs

<div style="border: 1px dashed" {{on 'click' (fn @onSelect @name)}}>
{{this.isActive}}
{{@name}}
</div>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}