I have to apologize as I wrote something not exact.
/pure/ has usually two different meanings:
m1) perfino,addirittura (Eng.: even, as in conceding something)
m2) anche (Eng.: also)
Usually (m1) is in exclamations.
Here the meaning is surely (m2).
How to get from this to the meaning of the complete sentence /entri pure/ ?
The long form wiould be:
a1) Lei puo` anche entrare (a kind invitation: not only you arrived here, but you may step inside)
which is transformed into:
a2) Lei puo` pure entrare
and shortened into a collection of set phrases (some I wrote in my previous answers) of which one is
b) entri pure.
(which, as I wrote before is a kind invitation, now I hope I gave you the reason).
By the way, don't think that 'imperativo' is only related to giving orders,
it can be very friendly, but this precise subject is not in the original question.
For the ones who strongly disagree with my opinion, they have every right to do so,
I will be interested in their intelligent remarks,
or /pure/ in hitting a new records of minus signs.