Wondering if....

[eL_]

... is there an add-on who limit the view of a post for unregistered users?
I mean to give them the option to register to read a thread?

 

[Old Nick]

There is a Resource module with an integrate search engine here : https://xenforo.com/community/resources
And if you use it you can find something like this :

Limited Guest Viewing Pro - Boost Registrations

Dramatically increase daily registrations by 200% to 300% Fully featured system to limit the number of content pages your guests can view before being forced to register. It also supports a nag system to warn users that they're on a limited...

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