Entry into the four house common rooms generally requires a password or a performance-test of some sort dependent upon that year or that specific month's security measures. For a canon example, it is generally understood that Ravenclaw utilizes different riddles that must be answered correctly before entry.
In order to obtain which methods are being used for any particular Hogwarts house in our world, I would ask the Head of House.
Now, unless one of these methods includes the pricking of a finger and the sampling of blood, or something potentially linkable to the true nature of a person entering the common room, then a good deal of these 'security' measures should be thwartable.
The absolute easiest way to sneak another house member into a common room, I would think, would be to simply invite them in; standing side by side with them, and performing whatever task is required before entry. If however, the portrait or suit of armor, etc. posing the security test also chooses to halt students from other houses, regardless of correct answers, you might have to consider a change of wardrobe or employ more advanced alterations.
Anything more advanced than wardrobe-changes might have to be run by the Head of House of course!
Hope that answers your question.
