Let's keep it simple:
- Beside means next to.
- Besides means also or in addition to.
One is just a preposition and one is a preposition and an adverb. Can you guess which one is just a preposition? (Psst. It's "beside.")
Example
Two people besides my sister were wearing flamingo earrings.
OR
Two people beside my sister were wearing flamingo earrings.
Which one's right?
Trick question, sucker. They're both right. But be careful because they mean two totally different things. The first sentence, "Three people besides my sister were wearing flamingo earrings," means "Three people in addition to my sister were wearing flamingo earrings." The second sentence, "Two people beside my sister were wearing flamingo earrings," means "Two people next to my sister were wearing flamingo earrings."