Biblical Foundation for Membership?

Rather than addressing the question of "membership" directly with a scriptural reference, I would like to suggest that there is a problem with the "Safe Church Policy" if a principal requirement is "membership in the church." What constitutes membership?

1. baptized;

2. completed profession of faith, but under 18;

3. active professing member, over 18;

4. inactive professing member; or

5. lapsed member.

Is a baptized child a member in the sense of being an "active professing member," i.e. one who has made profession of faith; has full voting rights at congregational meetings; can stand for office, etc. Do members in #2. & 3.- #5. meet the former criteria, even if they are membership categories. One might still want #2. - #3. to participate in the life of the church where the Safe Church Policy applies.

Secondly, can you think of a situation where someone might be worshipping with you for a short or longer period from another denomination who holds their membership where enfolding them into the membership of the Body of Christ on an interim basis only makes sense.

A Safe Church Policy should non-discriminatory in applying to members and non-members alike. 

I hope this take on the issue is of some assistance.

PS Maybe there's a deeper story behind the stance on membership. Personally, I know of someone who did not transfer their membership from their previous church due hurts arising in the former church and saw membership as problematic but wanted to be part of a worshipping community. 

I am not sure there is a simple scriptural/biblical response to this. I say this because the nature of the church changed during the apostolic era and continued to change after.

"Church" is a very poor translation of the word ecclesia which literally means "those called out" and came to refer to a community of individuals who were, in some manner, separate and distinct from others. Contrary to what we have been lead to believe, the ecclesia/church was not founded on the day of Pentecost. It was founded by Jesus when he called the first disciples and they essentially left everything to follow him, both literally and figuratively.  There was no particular requirement, simply that they tag along, learn from him, and seek to model themselves after him. This was what it meant to be a disciple.

That was when Jesus was out and about. Once he was (physically) no longer present, others essentially took on his teaching and when they passed, others took on their teaching. This meant that individual thoughts and responses came into play and the simple following of Jesus was lost to an increasingly complex and bureaucratic organization where the ministry of the many became the ministry of the few, namely the clergy. It also necessitated or at least invited a distinction between membership in the ecclesia/church universal and membership in a particular congregation or denomination with its distinctive beliefs and practices. 

Some congregations have addressed this by acknowledging everyone as members but requiring an understanding and commitment to their particular beliefs and governance before assuming a leadership position.