YOU SAID:
i would stay with my bestfriend
INTO JAPANESE
私は私の親友と一緒にいるだろう
BACK INTO ENGLISH
I will be with my best friend
INTO JAPANESE
私は私の親友と一緒にいます
BACK INTO ENGLISH
I am with my best friend
INTO JAPANESE
私は私の親友と一緒です
BACK INTO ENGLISH
I am with my best friend
That didn't even make that much sense in English.