Please read pool school. Top of this and every page, right side, white button.
Start with the ABC's of Pool Water Chemistry. It explains it all.
FC is free chlorine. TC is total chlorine.
Do you have a quality test kit? That's your second order of business after reading pool school.
Take a read through pool school before you let people swim. You must have chlorine in the pool, swimmers or not but especially if people are going to swim. You absolutely must disinfect the pool, with some form of chlorine. Post back with a full set of test results so we can help you. If you don't have a full FAS-DPD test kit, your local pool store can do a free test for you. Don't buy anything from them, just come back here and post the results. We'll guide you from there on out.
Can you tell us more about the pool? If you list your pool specifications in your signature line it's much easier for us to help you.