There are several options to assign a card:
Assign Card from the None field
If a card is not assigned upon card creation ->click on the None field of the card to open a dropdown menu with all users assigned to the board. Click on the username you want to assign to the card.
Assign Card from the Context Menu of a Card
Click on the context menu of the card -> select Assignee -> a dropdown menu with the users assigned to the board will open. Click on the username you want to assign to the card.
Assign Card from the Card Details
Open the card -> Click on the Assignee field to open the dropdown menu with the users assigned to the board. Click on the username you want to assign to the card.
Is it possible to assign multiple users to one card?
In Kanbanize, you can only assign one person to a card. The reason you cannot have more than one assignees is that collective responsibility bears the risk to turn into collective irresponsibility. Therefore, there can be only one owner of the work item, but there is an option to add more participants by adding a custom field of type: "contributor".
Add a Custom field of type Contributor (here can find a step by step tutorial on how to „Manage Custom Fields“). Open the card and click on Add field. Click on the contrubutor custom field to add it to the card. Click on None and select from the dropdown menu, the user you want to assign as a contributor to the card.
Subtasks of a card can be assigned to different team members than the official assignee of the card.
Open the subtasks of the card by clicking on the Subtask icon or by opening the Subtask tab of the card. Click on the assignee of the subtask and choose a user from the dropdown menu.