1 Chronicles 29:20
And David said to all the congregation, Now bless the LORD your God. And all the congregation blessed the LORD God of their fathers, and bowed down their heads, and worshipped the LORD, and the king. And David said to all the congregation, Now bless the LORD your God. And all the congregation blessed the LORD God of their fathers, and bowed down their heads, and worshipped the LORD, and the king.
The call to bless comes after the people have willingly given their offerings for the temple. It comes after David's prayer of thanksgiving. The whole congregation joins David in blessing the Lord, the God of their fathers. They bow down their heads as an expression of reverent worship. That worship is directed to both the Lord and the king.