What is a "proper person"? Is it a person who has manners and doesn't burp in public? Is it someone who is richer, more psychic, more powerful or academically accomplished? I don't think these have anything to do with the idea of proper personhood in Wicca.
Just so we are al aware, the term "proper person" does not belong solely to Wicca. As a matter of fact, we see the term used in English, Scottish, Australian and New Zealander law jargon in a number of ways from legislation to maritime regulations. In these cases, we often see the phrase as "fit and proper" and it can refer to people, states of readiness and repair, and a number of other things. There is even a test which determines how well one adheres to the accepted definition of the term which is can be administered to applicants to various jobs. The Masons also use the term Proper Person to indicate someone who is well suited to an office or job.
I believe that Wicca may have adopted the term Proper Person from the Masons as it uses the term in a similar way to indicate a person who is well suited for initiation. There is another layer to a proper person in Wicca, however, which is often felt rather than empirically determined. Consider for a moment the following definitions of the term proper from the Webster's Dictionary:
b : belonging to one : own
2 : belonging characteristically to a species or individual : peculiar
These definitions would refer to the word proper in its use as the root of the word property; that which is peculiar to or belongs to, or is owned by someone or something. It is not that the Wica believe that proper people are those who are literally owned by someone or some group, but that they belong to the Craft in general and to a specific tradition or practice in particular. Gerald Gardner recounted the tale of his meeting his initiators and their belief that he had been a member of the witch cult in another life. They said that since he belonged to them in the past, that he should return to them and join them once again. This is a belief that is still widely held in the traditional Wiccan community today. It is an explanation of the feeling of kinship which is sometimes felt upon meeting a potential student. It speaks to the feeling of having known someone very well in the past even though you have never met them.
Wicca is concerned with the common usage of the word proper. We do want to be sure we are initiating and training the people who are going to be good for our groups and traditions. We want to maintain our own status as proper people by keeping our oaths, stewarding our traditions with care, and by working to recognize those who the gods put in our path which are proper to the Wica and assist them (when they ask) to return to the practice. Have a look a the story which appears in the back several editions of Gardner's Meaning of Witchcraft. You will find evidence of what I have spoken of here.