Can not assign Task or User Story, to project Members in Azure DevOps

I am pretty new to DevOps. I've created a project in Azure DevOps and then invite some of my colleagues to project using the Invite button.

I can't assign anyone to the board Tasks or User Story. When I navigate to "Assign to" menu, the drop down is empty and there is the only option Unassigned.

enter image description here Where did I do wrong? please help.

Thanks

How do you assign tasks to your team members in resource list?

Assign Resource using Assign resources dialog. Select the desired task in the grid then go to menu Team - Assign - Resource. In Assign Resource dialog, select the desired resource and click on Assign button. Set the resource units or the number of items.

Can you assign more than one person to a task in Azure DevOps?

No. Work items can only have a single assignee. We work at the PBI level and will create duplicate tasks for each person that worked on it.

Enter to a work item and in the "Assigned To" filed start to type a username and press enter, it should identify the name and complete it and you can choose the user.

It's really bad UI/UX...!