Additional team and channel-related features and issues within Microsoft Teams for business
Hi klsokoli,
When you create a new plan in Planner, an Office 365 group is created alongside. This group can be viewed in OWA, OneDrive, so all the conversations in Planner are available in Outlook 2016, Outlook on the Web and the Outlook Groups Mobile Apps.
If you have created a private plan in Planner, then there isn’t any member added. To add the members, you must select the ellipses and click Members (screenshot 1), this will take you to OWA (screenshot 2) and here you can click Add members to add the members to the group.
Screenshot 1
Screenshot 2
Reference: Add and remove group members in Outlook
After you’ve added the members, you’ll see the drop-down list of the existing members once you try to assign a task.
When a new distribution list is created, we add members manually one by one, in the same way members must be added one by one in Office 365 groups.
To know more about Office 365, please read the following articles:
Why you should upgrade your distribution lists to groups in Outlook
If you wish to upgrade your distribution list to an Office 365 group, then please read the following article for detailed steps:
Upgrade distribution lists to Office 365 Groups in Outlook
Let me know if have any other doubt.
Thanks,
Neha