Hi Ryan-Mitchell,
Welcome to the Answers community.
From your post, we understand you would like to assign multiple tasks to the same user at once. In Microsoft Planner (https://tasks.office.com/), unfortunately it is not possible to select more than one task.
We understand this is a very important and useful feature, it would be very convenient if we can assign tasks in bulk and in this regard, we would like to invite you to provide your feedback in Planner UserVoice where other customers have requested the same feature. UserVoice is the best way to give the feedback to Product Group as it’s actively monitored by them and many new features have been implemented from our customers feedback.
There is, however, a workaround, if you are using Microsoft Teams and have the plan added as a tab, go to the 'List' view. From the List view, you can select multiple tasks by holding down the Shift or Ctrl key. Then you can select Assign at the top to bulk update user. You can also select Progress, Priority, Due Date, and Bucket to bulk update.
We appreciate your understanding and co-operation.
Kind regards,
Neha