How to enable 'external access' for collaborative work in Microsoft Teams?

How to enable 'External Access' in Microsoft Teams Admin Center
Browseto: https://admin.teams.microsoft.com/company-wide-settings/external-communications to enable external access.
Or follow the instructions in the video by going to: https://admin.teams.microsoft.com/ or https://admin.microsoft.com/
Users can communicate with other Skype for Business and Teams users.
Users can communicate with Skype users.
This may take up to 24 hours after activation.
Activate "Allow all external Domains" in the tab: Teams and Skype for Business users in external Organizations

How to enable guest access in Microsoft Teams Admin Center
Navigate to: https://admin.teams.microsoft.com/company-wide-settings/guest-configuration
To enable Remote Access for Microsoft Teams these settings are required.
Allow guest access to Teams - ON
call - Make private calls - ON
Meeting - Allow IP video - ON
Screen sharing mode - ON
Allow meeting now - ON
This may take up to 24 hours after activation.

External identities - Setting up external collaboration settings in Azure Active Directory (AD)
By default, each tenant set as shown below. Within this external identities panel, you can make adjustments to include less or more security.
Navigate to the Azure External Identities panel ->https://portal.azure.com/#blade/Microsoft_AAD_IAM/CompanyRelationshipsMenuBlade/Settings
Guest user access restrictions
This setting determines whether guests have full access to list all users and group memberships (most inclusive), restricted access to other users and memberships, or no access to other users and group memberships, including groups of which they are members (most restrictive).
Guest invite settings
This setting determines who can invite guests to your directory to collaborate on resources protected by your Azure ADsuch as SharePoint-sites or Azure resources.
Collaboration restrictions
Allow or block specific domains.

Block guest access to specific Microsoft Team sites
If you have enabled guest access but do not wish to welcome guests in some teams you can disable it in the SharePoint panel.
1. Use the SharePoint administrator panel
Browseto the SharePoint administrator panel from the admin centre. https://admin.microsoft.com
Select Sites, Active Sites
Select the desired team
Click on policy
External sharing

2. Activate "Only people in the organisation".
Select between these 4 options: Only persons in the organisation
Everybody
New and existing guests
Only existing guests
Only persons in the organisation

Other practical Microsoft Teams blogs
How to prevent users from deleting channels, tabs & apps in Teams
Communicating with external users in Microsoft Teams - meetings
How to share files in Microsoft Office 365 and Teams - 8 scenarios
How to import users and guests in CSV in Office 365 or Azure AD?
Adding multiple users to a Team at the same time
Dag Jasper, nice description of the settings, but what I want is this: I want to prevent guests from being added to a team at some teams , but I do want members of this teams to be able to share files with external mail addresses. When I turn off external sharing, to prevent guest access, team members cannot share files either. How can I set up that guest access is possible at the organisation level within teams , turn it off for some teams , but that they can still share files with external mail addresses within this teams ?
Hi Peter, thanks for the compliment! You can close Teams and not allow guests to access it. I think to keep it clear you better set up teams with guest access and teams without guest access. important or 'internal' sensitive data can for example be without guest access. Document sharing on document level is a possibility but is not very practical today from teams even. Then you better work via the fat-client of for example Word OR SharePoint online . The experience (and possibilities) is not the same within Microsoft Teams than in SharePoint or Word. There is a solution for that here: https://www.microsoft.com/en-us/microsoft-365/roadmap?filters=&searchterms=Teams%2Csharing
Sharing files in Microsoft Teams can be done via this blog: https://365tips.be/bestanden-delen-in-office-365
Good site with many tips indeed!
I have added guests to a team but they cannot see the documents. The internet users, however, can see them. What am I doing wrong? Hope you can help me.
Hi Dave, thanks! The ultimate test is always in a browser or a 2nd computer. Do you have this problem with several guests?
Hi Jasper, I can't seem to add guests to a team. I have guest access turned on in the admin but I still can't add guests. Do guests have to have an account before they can be added to a team?
Hi, it can take up to 24 hours before you can add guests after activation in the admin panel. After that it should work. Have you tried adding them in the general channel?