Guest Access in Microsoft Planner
Guest access in Microsoft Planner allows us to collaborate and participate with people from outside of your organization. The guest users can create and delete buckets as well as tasks, edit task fields, and edit the plan name.

Guest access in Microsoft Planner allows us to collaborate and participate with people from outside of your organization. The guest users can create and delete buckets as well as tasks, edit task fields, and edit the plan name. If you grant additional permissions to the guest users, they can also attach a file or link to a task. Guest users will also receive some of the notifications which are associated with your plan.
How it works
The planner builds on the Office 365 groups for the guest access. If Office 365 Group's guest user access is turned off, you are not able to add guest users to plans. You have to enable it from Office admin.
Step 1 - The group owner adds a guest to the group.
Open Outlook for Windows.
Under Groups in the left pane, select the group.
On the Groups ribbon, select Add Members.
Enter the email address of the guest in the Add Members box, and press Enter.
Select OK.
Step 2 - The invited guest will get a welcome email from the group. If they do not
have the guest account with the organization, they need to accept this
Invitation to create one. In the welcome email, go to Read group files, select
the Access files link and follow the instructions to create a guest account.
Step 3 - To access a plan, the guest users either need to use a specific plan URL or go
to https://tasks.office.com/[organizationdomainname]. The general
Planner URL, https://tasks.office.com/, takes non-guest Office 365 users to
Planner, but they don’t know from the guest user name which organization
they want to sign in to.
Step 4 - The guest can leave the plan at any time using a link at the bottom of each
email and calendar invitations.
Permissions:
The below table summarizes what group owners, plan members, and guests can and can't do.