This feature has evolved quickly, from being limited to 50 external users that could do very little to 5000 users that when added to customer groups can do quite a lot.
Still the duration of the run hasn't changed. Once the invitation accepted the external user can do his stuff for 30 days, after that the invitation will expire and a new one will need to be created.
Keep in mind that if you want to track these users activities for your activity history books, there users had better have proper names since their first creation. The reason being is that once created, even if the user changes his name later on it will not help. That first name will be there for good.
"Since Deleting the External User from SPO won’t make it appear updated in SharePoint Online either. All that does is remove the permissions for that user. It doesn’t actually remove them from SharePoint Online Directory Services. So when an administrator recreates the user, technically the user already exists and they are just giving it permissions again. If the administrator wants this much control over a users display name, they should really grant them a SharePoint license. That is their best solution."
Now if you are trying to but is unable to get the Site Sharing feature to work, please, allow me to give you a piece of advice - Try Starting it all Over Again.
By that I mean, turn off External Users site sharing at the Portal level. Be aware that if the site is approaching its usage limit, as in the PNG below, you might encounter some problems farther down the road, when you start to invite your external users:
Next, turn the feature off in the site collection features list:
Then, just log off.
Now log back in and start configuring it over from the beginning, that means reversing the steps described above or Starting it all Over.
As a solution this sounds king of silly, I know, but if I had an euro for every time I saw this work and fix the problem, well, chances are... I would still be writing this post.
Best regards from the cloud,
Helio de Andrade