Can I share Sharepoint site which is created by Planner to people outside my organization?

[Lawrence Lee]

The sharepoint site created by Planner is prohibited to share outside of my organization by default.

And Sharepoint admin cannot see the site collection created by Planner. 

How do I change the setting of the site to share outside organization? 

[#2]

Yes,
You need to enable sharing capability of the site by using PowerShell:
$sUserName="<Office365Admin_Or_SPO_Admin"
$sMessage="Type your Office 365 Credentials"
$Office365Cred=Get-Credential -UserName $sUserName -Message $sMessage
$sSPOAdminCenterUrl="https://<Office365Domain>-admin.sharepoint.com/"
Connect-SPOService -Url $sSPOAdminCenterUrl -Credential $Office365Cred

$sSPOGroupSite="https://<SPO_Site_Where_External_Sharing_Needs_To_Be_Enabled>"

Set-SPOSite -Identity $sSPOGroupSite -SharingCapability ExternalUserSharingOnly

[#3]

@Juan Carlos González Martín. Thank you. It's working now. 

I added my personal MS account to the sharepoint site created by Planner successfully.

However, I tried to add another personal MS account but it failed. 

It shows below message. However, I checked users in our Office365 admin. My 2nd personal MS account is on it. I have no idea how to fix it. 

--------------------------------------------------

We're sorry, but [my 2nd personal MS account] can't be found in the gotrust.sharepoint.com directory. Please try again later, while we try to automatically fix this for you.

Here are a few ideas:

Click here to sign in with a different account to this site.
This will sign you out of all other Office 365 services that you're signed into at this time.

If you're using this account on another site and don't want to sign out, start your browser in Private Browsing mode for this site (show me how).

If that doesn't help, contact your support team and include these technical details:

Correlation ID: d9720f9e-a0a3-4000-9fd8-a64f69b6b531

Date and Time: 2017/8/15 上午 01:26:58

URL: [Test Sharepoint Online Site]

User: [my 2nd personal MS account]

Issue Type: User not in directory.

[#4]

I have seeing this in some cases...I would recommend to re-add the user again in the site and try to access the site using an inprivate session