#1 - You're not going to attend a church where you don't know anyone. But, you're not going to make any deep meaningful friendships by just going to the Sunday AM service. You can develop new friendships in a small group though.
#2 - You're not going to get everything you need in your spiritual formation by just attending the Sunday AM service. What is presented there is for the whole community of faith. But a small group can address specific questions and take the time you need to grow in your relationship with Jesus.
#3 - You'll understand your church better, when you see it in action. Sure the Sunday AM service is great. But small groups reveal who and what a church is up close and personal.
Different churches approach how they do small groups differently. However they do it - find a class and attend for a few weeks. If you like what you experience, then stay. If it isn't a good fit - try a different class. Whatever you do - GO!