In Office 365 you can create Office Groups. An Office Group is a solution where each member can participate in conversations, schedule meetings, share files and notes and even initiate a Skype for Business voice and video call for urgent real-time decisions. You can use Office Groups to collaborate with external users. They are called ‘guests’.
To enable guest access in Office Groups you need to set the option in the admin section of your tenant. There is an excellent article that describes these steps.
What happens when you invite a guest?
You add a guest by entering his email address. Then this flows starts:
The guest gets an email in his mailbox:
That email says that the guest is now member of the group and contains a link to the group files and a link to start a conversation. The guest does not have access to the whole list of conversations (so he’s unable to see a historic overview of the conversations in the groups when he is invited later) but he can send an email to the group.
When the guest wants to access the files he must create an account:
He can choose a password and must provide some basic information about himself.
A verification code is send to his email address to make sure he is the right one.
At the end of the process he can access the underlying SharePoint team site of the group where he can access the document library with the files.
The link to the group will not be visible in his Outlook desktop application. The user must bookmark a link to the group (or save his email).