Are Christians allowed to join a Sorority or Fraternity?
I believe before one joins any 'group' be it a sorority or fraternity, they should look at the purpose and intent of that group and their own reasons for wanting to join. There are many scriptures that talk about how a Christian should live their lives. The Bible says we should make it clear to others our intentions as to how we think of things that are not in line with God's will. Now that does not mean, we should lock ourselves up and never associate with anyone that is not in Christ, but be transformed in our thinking so when questions like this come up, we know how to deal with these issues;
"Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is-his good, pleasing and perfect will." Rom. 12:2
Again, some interpret this to mean we should have nothing to do with the world. That's not what the Bible is saying. We want to be a 'preserver' of good and show others how doing the will of God is far better than 'just going along';
“You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men. You are the light of the world. A city that is set on a hill cannot be hidden." Mat. 5:13-14
Salt, is a preservative and showing others your Godly life is being a 'light' to the world.
I do not want to imply that all sororities and fraternities are evil. However unfortunately a lot of times these organizations are in existence for the 'partying'. The activities can even become dangerous. There are many unsuspecting young adults that have their lives taken away because they just tried to 'fit in'. If you say 'No' to these invitations, you are mocked. But we must remember the Bible warns us of these things;
"For we have spent enough of our past lifetime in doing the will of the Gentiles-when we walked in lewdness, lusts, drunkenness, revelries, drinking parties, and abominable idolatries. In regard to these, they think it strange that you do not run with
them in the same flood of dissipation, speaking evil of you." 1 Peter :3,4
We don't need to be concerned with 'what they think', because a true Christian knows in his heart what God requires of him or her;
"But even if you should suffer for righteousness' sake, you are blessed. "And do not be afraid of their threats, nor be troubled. But sanctify the Lord God in your hearts, and always be ready to give a defense to everyone who asks you a reason for the hope that is in you, with meekness and fear;" 1Peter 3:14,15
So, you need to ask yourself, what are my reasons for wanting to join and what is the intent of this group. Then pray for guidance. The Holy Spirit within every Christian will never lead you into wrongful temptation, but will deliver you from the evil ones doings.