There are several ways to assign a card:
1. Assign a Card from the None field
If a card is not assigned upon card creation -> click on the None field at the top right side of the card to open a drop-down menu with all users in the board -> select the username.
For convenience, your username always appears as a second choice.
2. Assign a Card from the card Context Menu
Click on the card context menu -> select Assignee -> a drop-down menu with the users in the board appears. Select the username you want to assign to the card.
3. Assign a Card from the Card Details
Open up the card -> click on the Assignee field to open the drop-down menu with the users in the board. Select the username you want to assign to the card and update it.
4. Is it possible to assign multiple users to a single card?
In the default Assignee field of the card, you can assign only one user who is the owner of the task (1).
The system allows you to add more card participants/contributors via the custom field of type: "Contributor". This custom filed will produce a dropdown of the internal users in the account that can contribute to a card (2).
To visualize the second assignee filed on the card -> you have to add a Custom field of type Contributor (follow the link to find a step by step tutorial on how to „Manage Custom Fields“).
When you add custom fields to a Kanban board, they appear as choices in the Custom field section of each task. In case the custom field is not mandatory -> click on Add Field -> select the custom field and it will appear on the card.