Guest Lecturer title
I pretty strongly suggest that you don't claim a title of any kind unless it is formally conferred. That is, if you want your CV to be credible. 

But there is a difference between "University Guest Lecturer" and "university guest lecturer". The first seems to be a title, but the latter is just descriptive. You can certainly describe what you do in a CV without making any claim to hold a position that the university hasn't actually conferred. 

Caution is best, I think, in all such situations.