'우리' and '저희' means totally same.
But the difference is coming from whether other party is higher than our.
If you're saying to senior/older people or formal situation you should use '저희'.
ex) 저희는 한국사람입니다. - We are Korean.
You are same qualification(like friends) or older than other party, you should use '우리'.
ex) 우리(나라)는 미국과 동맹국가입니다. - Our Country is an ...