why is 'se' used in this sentence? could it be omitted or does 'une' require it?
When it means "each other, a group, etc." use unir-se (reflexive), then we have "se unem".
But would the sentence have the same meaning without the word 'se'?
no, when you use "se" means the capitain is "in one side". If you don't use "se", then the capitain join things/people. For instance, "o capitão une o útil ao agradável".
Think I got it, thanks.