You have gotten some very good advice from AXiDGirl10!!
Informal recruitment is usually very different from formal, and can allow you to see chapters and sisterhood in an entirely new light. You may find that you fall in love with the Sigmas, or you may discover that the Gammas or the Kappas are your true home. Keep an open mind, accept every invitation you can, meet as many sisters as possible, look to see who else the groups are recruiting (your new member class).
You will never know why you weren't invited to pref with Beta, but please don't let it hurt your confidence or your self-esteem. It may be that they sensed your enthusiasm for Alpha. Every sorority wants girls who want THEM. Or, they may have a history of large returns to pref parties and had to make some heavy cuts in order to meet panhellenic return number guidelines. Perhaps the sisters who knew you best were not very vocal in MS----you will never know. Cuts always hurt, no doubt about it, but they are not always a sign that the sorority didn't like or admire you. Most sororities see many potential members as desirable--they just can't invite them all back.
Another word of caution--partying and clubbing with people doth not a sisterhood make. You might find that you enjoy socializing and clubbing with a certain group, but that doesn't necessarily mean you will enjoy living with them, working on philanthropy and MS with them, making difficult decisions with them, perhaps knowing them for the rest of your life (many of my chapter sisters are now in alumni groups and on chapter advisory boards together and their daughters are chapter sisters in the same house. For many sorority women, it is truly a lifetime commitment that goes far beyond social compatibility--although that is certainly important
.)
Good luck with informal!! I hope you have a wonderful experience. You never know where it will lead.