AR 600-20 spells out chain of command and when someone is in charge. For example, if an E-6 is senior in grade by date of rank he can rate you and counsel you. Typically, it will be from the perspective of being in charge of you. For example: If the Platoon Sergeant is an E-6, and the Squad Leader is an E-6, the Platoon Sergeant can counsel the Squad Leader – and even rate him – as long as he is senior in grade (See AR 623-3 on Rater requirements).
Another action might be that the Platoon Sergeant is junior in grade to the Squad Leader. In this case the Platoon Sergeant can counsel the Squad Leader, but not be his rater.