Compelling Reasons for Attendees to Become Members?

Some other reasons: 

If they participate in activities maybe one could be a study on what it means to be Reformed, and within that framework you take an in depth look at church membership as a faith commitment. One way to emphasize the membership step is by looking at what some liturgical forms used in profession of faith include, such as commitment to the Scriptures, to love, to the church, but also denying of self, renouncing Satan, etc. 

1. If they are baptized members, profession of faith is their response to God's promise in baptism. In the Reformation time baptism was a way of clarifying if this baby is a Catholic of a Protestant. Today the infant baptism initiates you into this covenant community and a response to God's promise is expected if the baptism is properly understood. 

2. In membership - which is a profession of faith - they are confessing their faith in a confessional church. The CRCNA is a confessional church and by joining the church publicly and officially they commit to abide by the faith commitments of this denomination. It is a privilege to know where you church stands clearly on a few important faith doctrines, etc. 

3. Becoming church members in a CRCNA also demands a responsibility from them - they make the same promises that the others in the covenant community have made. They will nurture others in faith and they would submit themselves to being nurtured and also held accountable for their faithfulness to God and the Word. 

4. Reading the NT is a great way to look at how the church grows. In Acts the church is exploding and expanding. It happens so quick that in Acts 15 there is an issue the church has to solve, related to doctrine and requirements - and there is a synod gathered in Jerusalem. If they are interested in how the church works as institution, invite them to participate in a classis meeting, or to sit at synod - these are open meetings. Or, invite them to look over some of the CRCNA materials, synodical records, etc. For some these may be boring for others interesting. 

5. Membership in the church in the USA is voluntary. You are a baptized member though, if you were baptized as infant - that's a great start. If not a baptized member, then the emphasis should be on baptism, as a sign of obedience to Christ - that can only be done - in a confessional church - in the worship service of the congregation. And that's another great connecting point. Ecclessiology is something that may be the last thing they want to look at, but that what they are weak in, in my view. Any way you can find to teach them about the church is going to help. Also, most of the former Christians in Western Europe started to doubt and disregard the need for the church. Today they don't go to church, maybe only 4%? and that's in most cases a state church, that is, not a voluntary church (like in the USA) where if you don't go and offer your worship and money the church closes its doors. The Eastern Orthodox Church is mainly the same - struggling with high nominalism - people don't go to church but the church continues to function as an institution. Here is not the same, the US is a voluntary church. If all people would follow their example - you can say that we assume they are right not to join - how would the church look like?

If they have already made profession of faith that should have granted them membership immediately. 

If not - you can revisit their profession of faith. I'm interested to see what happens with them, what moves them. I'm praying for you and for them.

Grace, Daniel