• Create a group in Outlook
• Join a group in Outlook
• Leave or unsubscribe from a group in Outlook
• Add and remove group members in Outlook
• Make Microsoft 365 groups public or private
• Delete a group
• Guest access in Microsoft 365 groups

Please see this Microsoft article on how to create a Group in Outlook desktop or web versions. 

When you join a group, you gain access to the group mailbox, calendar, OneNote notebook, and team site. Permissions are taken care of automatically so you don't need to worry about missing any meetings or conversations.

Please see this Microsoft article on how to join a group  using the Outlook desktop, web or mobile apps.

When you leave a group, you give up your access to the group inbox, calendar, OneNote notebook, and library. The only way to regain access is to rejoin the group.

Please see this Microsoft article on how to leave or unsubscribe from a group  using the Outlook desktop, web or mobile apps.

Any member of a group can invite others to the group, but only the group owner can remove them. By default, the group owner is the person who created the group but others can be assigned this role as well.

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Make Office 365 groups public or private

Please see this Microsoft article on how to make groups public or private in the Outlook web version.

Please see this Microsoft article on how to delete groups in Outlook desktop or web versions.

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Guest access in Office 365 groups

Guest access in Microsoft 365 Groups enables you and your team to collaborate with people from outside your organization by granting them access to group conversations, files, calendar invitations, and the group notebook. Access can be granted to a guest; including partners, vendors, suppliers, or consultants, by any group owner.

Please see this Microsoft article for an explanation of how guest access (external collaborators) works.