보고싶어요 literally means 'I want to see you', but it is usually used when you want to say 'I miss you'. 그리워요 is hardly used verbally. It can be used in writen Korean in a different form which is 그리워하다. For example, 그녀는 그를 그리워했다(She longed for him). Hope this help~

the first one means I miss you (bogoshipoyo) and the second one means I am longing for you (guriwo). Somehow they are the same, but it depends on how you use it in a sentence.