So I found the commands in Exchange Powershell to add a user to have rights to a shared mailbox. Then I wrote it out as a script.

Import-Module ExchangeOnlineManagement
$AdminAddress = Read-Host -Prompt "Enter your admin e-mail address"
Connect-ExchangeOnline -UserPrincipalName $AdminAddress -ShowProgress $true
$SharedCalendar = Read-Host -Prompt "E-mail address of shared mailbox"
$RightsRecipient = Read-Host -Prompt "E-mail address of person getting rights to calendar"
$RightsLevel = Read-Host -Prompt "Enter either Editor or Reviewer"
Add-MailboxFolderPermission -Identity $SharedCalendar:\calendar -user $RightsRecipient -AccessRights $RightsLevel

Now, this works… assuming you don’t have any typos and you have all your ducks in a row. HOWEVER, I wanted to cover most of my bases, and demonstrate to my team how to write a more ‘robust’, error-checking script with documentation, to be used as a reference for future PS scripting. So I came up with this-

# JMarcum - 2024-Jan-05  // Always sign and date your code, and
#            add revision notes below the 'Purpose' section.
# PURPOSE:
# Grant requested access level to shared calendar in O365 to specified user

###  FUNCTIONS
# Validate text choices for granting calendar rights
Function ValidateOption {
  Param (
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]
    [ValidateSet('E','R')][string]$RightsLevel
  )
}

# Validate e-mail address as actual mailbox
function Test-ADUser {
  param(
    [Parameter(Mandatory=$true)]
    [ValidateNotNullOrEmpty()]$mail
  )
  try {
    # Cast address as mailaddress and see if 'valid'
    $null -ne ([mailaddress]$mail)
    # If e-mail address format is valid, see if user in forest (or the scope of account you are running this as)
    $null -ne ([ADSISearcher]"(mail=$mail)").FindOne()
  }
  catch {
    return $false
  }
  finally {
    $Error.Clear()
  }
}

###  CODE
# Break if error
$ErrorActionPreference = 'Stop'

# Connect to O365 using admin credentials
try {
  Import-Module ExchangeOnlineManagement
  $AdminAddress = Read-Host -Prompt "Enter your admin e-mail address"
  Connect-ExchangeOnline -UserPrincipalName $AdminAddress -ShowProgress $true
}
catch {
  Write-Host "Run the following commands in PowerShell before running this script:"
  Write-Host "Install-Module -Name ExchangeOnlineManagement"
  Write-Host "Install-Module -Name ExchangeOnlineManagement -Scope CurrentUser"
  Exit
}

# Get addresses of calendar to share and user to give rights to
# Shared Calendar e-mail address
$SharedCalendar = ''
do {
  try {
    $SharedCalendar = Read-Host -Prompt "E-mail address of shared mailbox"
    if (($SharedCalendar -eq '') -or ((Test-ADUser $SharedCalendar) -eq $false)) {
      throw
    }
  }
  catch {
    Write-Host "Please check address and re-enter"
    $SharedCalendar = 'invalid'
  }
  finally {
    $Error.Clear()
  }
} until (Test-ADUser $SharedCalendar)

# E-mail address of person being granted rights to shared mailbox
$RightsRecipient = ''
do {
  try {
    $RightsRecipient = Read-Host -Prompt "E-mail address of person getting rights to calendar"
    if (($RightsRecipient -eq '') -or ((Test-ADUser $RightsRecipient) -eq $false)) {
      throw
    }
  }
  catch {
    Write-Host "Please check address and re-enter"
    $RightsRecipient = 'invalid'
  }
  finally {
    $Error.Clear()
  }
} until (Test-ADUser $RightsRecipient)

# Get requested calendar rights for user
$RightsLevelTest = ''
do {
  try {
    $RightsLevelTest = Read-Host -Prompt "(E)ditor/(R)eviewer"
    ValidateOption $RightsLevelTest
  }
  catch {
    Write-Host "Please type either E for Editor or R for Reviewer"
    $RightsLevelTest = ''
  }
  finally {
    $Error.Clear()
  }
} until ($RightsLevelTest -ne '')

# Assign correct value to rights level
if ($RightsLevelTest -eq 'R') {
  $RightsLevel = "Reviewer"
} else {
  $RightsLevel = "Editor"
}

# Grant requested rights to calendar
try {
  Add-MailboxFolderPermission -Identity $SharedCalendar:\calendar -user $RightsRecipient -AccessRights $RightsLevel
}
catch {
  Write-Host "Please re-run as account with O365 Admin rights"
}

With this code, I have accounted for a majority of the ‘oops’ that can occur in a script that calls various services. Yes, I could have gotten more involved in certain checks (like confirming Exchange Admin rights at the beginning), but this was more a “proof of concept”.

Hopefully it helps some people out there!