I had a request recently from a client of mine, who was not seeing all of the Office 365 Groups (that had Microsoft Teams instances) showing up in Outlook.
Typically within the Outlook client (and web version for that matter) in the left hand menu you see a 'Groups' section, with Groups that you have joined underneath it.
The client had tried all sorts of things within the Outlook client, including going into Manage Groups | All, and then rejoining each the groups (that they were in fact already an owner of). In some cases, when attempting to join the group, they would get the error 'Your request could not be sent. Please try again later'.
Their main 'All Staff' also would not show up, which meant that people couldn't easily create a company event (luckily I had the group calendar app on their Intranet home page, so they could add events that way).
I found the article https://petri.com/hiding-office-365-groups-exchange-clients which stated 'Office 365 will hide groups created by Teams from Outlook by default'. I understand that this may have been a strategy from Microsoft to stop the clogging up of Microsoft Groups showing up in Outlook, but my company was quite small and needed to show all of their groups.
To fix the issue, I first ran:
I then ran Connect-ExchangeOnline -UserPrincipalName "<email address>" -ShowProgress $true.
Once connected, I ran:
Get-UnifiedGroup -ResultSize Unlimited | Select DisplayName, Alias, PrimarySmtpAddress, HiddenFromExchangeClientsEnabled
This listed each of the groups, showing that in fact that many were configured to be hidden from the Exchange client (HiddenFromExchangeClientEnabled attribute).
I then ran Set-UnifiedGroup -Identity "<group name>" -HiddenFromExchangeClientsEnabled:$false against each group that needed to be exposed.
After exiting Outlook and starting it again, the list of groups then showed up. As a side benefit, as the All Staff group was now showing up, people could easily enter company events through the Calendar area of Outlook.