What is natural wording when you ask friends to eat?
When you serve simple lunch to friends, what do you say? Have it, please? Help yourself? Enjoy it?
Can I say "Help yourself!", even if it's not big lunch, just simple and a little quantity of food?

Question

What is natural wording when you ask friends to eat?
When you serve simple lunch to friends, what do you say? Have it, please? Help yourself? Enjoy it? 
Can I say "Help yourself!", even if it's not big lunch, just simple and a little quantity of food?

Answer

I agree with Darryl's comments. Definitely don't say "Have it, please". It sounds as if you know the food is terrible but you're begging your friends to eat it anyway.

A longer sentence could be, "It's not much, but I hope you enjoy it."

Answer

Help yourself.
or
Enjoy!