It seems the commandments were directed at Moses even though Aaron was present
Numbers 20:7 (KJV)
7 And the LORD spake unto Moses, saying, 8 Take the rod, and gather thou the assembly together, thou, and Aaron thy brother, and speak ye unto the rock before their eyes; and it shall give forth his water, and thou shalt bring forth to them water out of the rock: so thou shalt give the congregation and their beasts drink.
Moses strikes the rock instead
Numbers 20:11 (KJV)
11 And Moses lifted up his hand, and with his rod he smote the rock twice: and the water came out abundantly, and the congregation drank, and their beasts also.
The judgement on Aaron
,Numbers 20:24 (KJV)
24 Aaron shall be gathered unto his people: for he shall not enter into the land which I have given unto the children of Israel, because ye rebelled against my word at the water of Meribah.
Was Aaron as guilty as Moses?