To enable users to add guests within the tenant

Sometimes you need users to add guests on their own. Normally, this can only be done by Admins. To enable this:

1. Log into the Office 365 admin portal (https://admin.microsoft.com/)
2. Go to Settings | Org settings (left blade) | Security and Privacy (menu bar) | Sharing
3. Check Let users add new guests

To add a new guest user

In the Office 365 admin portal:

1. Select Users | Guest users
2. Click + Add a guest user.  You’ll be redirected to Azure
3. Select Invite user and enter their details. 

The guest user needs to accept the invite before gaining access to anything.

To add them to a Planner board, follow https://support.microsoft.com/en-us/office/guest-access-in-microsoft-planner-cc5d7f96-dced-4da4-ab62-08c72d9759c6