I have this requirement as part of a site that will be built specifically for external users in SharePoint Online.
It's basically an automation need for users to be granted access to certain areas of the site by adding them to appropriate SharePoint Online groups. Users should also be automatically created by a workflow when their details are picked up from a list.
I've done an attempt to do this in SharePoint Designer, but it turns out there's no such action in there.
A couple of alternatives exist - Nintex Workflow which I have in place and Plumsail Workflow Action Pack which I don't have in place but a quick overview can be found on their website.
With Nintex, there is the handy action "Office 365 add user to group", which is usually working well. For external users, however, I found it impossible to resolve them like any Office 365 user.
The only alternative I found so far is to create this user in the Office 365 tenant, assign a license to it (all automated through the workflow) and then add it to the group. Unfortunately, my requirements involve no licensing for such users, so I'll have to find another possible automation approach.
Has any of you managed to accomplish this? Would be happy to hear oppinions.