The default for a microsoft o365 group is to hide it from the global address list. Currently there is no way to change this in the GUI, you need to use a quick and easy powershell command.
First you need to connect to Exchange online
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection Import-PSSession $Session
Once you connect to exchange online you just need to update the attribute hiddenfromaddresslistenabled
Set-UnifiedGroup -Identity "Group Name" -HiddenFromAddressListsEnabled $false
That’s it once the GAL is rebuilt overnight, you should start seeing it in Outlook.