There are several ways to easily assign members to items in a collection. Assigned members will receive a notification when they've been assigned so that they know that they're responsible for the task.
Drag and drop
The fastest way to assign members is to drag and drop them. You can drag and drop a member from anywhere that a member's icon or initials appear: the member's menu, from your own user icon, from a member field in which they're already added as a member, or even from activities and comments!
Drag the member you want to assign an item to, and drop it onto the item. The item will turn grey when it's registered that your mouse is hovering over it.
This will assign the member to the first member field in the item (the member field shown highest on an open item). If no member field exists, a member field will be created when you add the first member. If you have an item detail panel open, you can drag and drop a member to a specific member field.
Search and add
You can also easily add a member from an open detail panel. Just click on "Add a member" under the member field you want to add them to and select them from the dropdown menu.