A simple sentence usually has one subject and one verb. However, a simple sentence can have a compound subject as long as it still has only one action.
Here are sentences with one subject and one verb:
John plays the piano.
Sally traveled to Europe.
Michael will meet us at the mall.
These next sentences have a compound subject:
John and his sister play the piano.
Sally and her mom traveled to Europe.
Michael and Rick will meet us at the mall.
Each of these is still a simple sentence. Even though each sentence has a compound subject, only one thing is happening in each sentence.
A simple sentence can have a compound subject.
▶Add another subject to this sentence so that is has a compound subject. Bob will join us for dinner. Did you add a name so that Bob is not the only subject? Good for you!