"I guess so" is a very useful phrase for reluctantly agreeing with someone. "Reluctantly" agreeing means agreeing but also showing that you're not completely happy to agree.

A: Please come. It'll be so much fun!

B: All right, I guess so.

A: Cool!

A: Would it be OK if I borrow your pink shirt for my date tonight?

B: Uh, I guess so.

When you say "I guess so" the stress should fall on the word "guess". The pitch of your voice should also rise on the word "guess" and then fall and rise on "so"