Tasks can only be assigned to active users of Claimable.
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To do so, just click on the task to open the edit page, click on "Change" under "Assigned To" and select a user.

If the user you would like to assign to the task is not on the list, you may want to invite him or her to use Claimable. To do so, just contact us or ask the Admin of your Claimable account.