1) our status as Christians,
2) our obligation as Christians
Our status, is that we belong to Christ and his body, the church.
Our obligation, is that we live in accordance with belonging to Christ and the church.
The bible tells us that we must recognize these things if we are to take communion:
ESV 1 Corinthians For anyone who eats and drinks without discerning the body eats and drinks judgment on himself.“Discerning the body” in this verse means “recognizing what characterizes the body as different.” or “recognizing our status and obligations as Christian believers” 
In the Bible times, some people took communion without recognizing their status and obligations, and God judged them, and some got sick, and some died.
ESV 1 Corinthians That is why many of you are weak and ill, and some have died.
Therefore, if we make sure that only members (people who recognize their status and obligations as Christians) take communion, then we protect people from getting sick and dying.
At the same time, we welcome visitors and new comers to our church to become members. The membership process is not difficult, and is designed to reflect what the bible says a Christian is supposed to do in a church.
 Anthony C. Thiselton, The First Epistle to the Corinthians : A Commentary on the Greek Text (