Yes Lowe's does assemble full sized grills for you for free when you buy the full sized grills.
Usually though Lowe's will already have assembled grills ready to take home so you usually do not need them to assemble any grill for you to take home.
However if they don't have an assembled grill that you want to buy then yes you can have the Lowe's employee assemble the grill for you at no extra charge.
Also if you live pretty close to the Lowe's store and the grill costs $499.00 or more then Lowe's will even deliver the assembled grill to your home for no extra charge.