I\'ve just joined my friend\'s Team Services account (free for the first 5 users account). If I visit the home page (https://XXXXXXX.visualstudio.com/Development/Development
In the Assign To dropdown list of a work item, it shows a list of most recently used users.
Because you have just joined VSTS and never assigned work items to others, you could only see yourself in the dropdown list. To assign your work item to your friends, you could search his username at Assign To textbox.