While it may not be quite so concerning if you are renting out an entire house with the yard to people, you do still need to give some thought to a grill and the possibility of them doing this at your rental property. Often, these gatherings mean alcohol. While we always like to think those we rent to will be responsible with our property, alcohol can make people do some pretty crazy things. You always want to make sure that, if nothing else, there is good renter's insurance on your property just in case something goes wrong while you are not living there.
This is especially true if you are only renting an apartment out. For example, the apartment that I rent has a small balcony outside on the second floor. The downstairs neighbors have a grill and they say that a few years ago we would have been allowed to have one as well. We are not, however, due to fire restrictions. There is a very strict ban on any second floor residents grilling something over an open flame, even on the outside balcony. While it would be nice to be able to set up a small campground grill, we are not allowed and must respect that.
Something to think about when you are writing up your lease is what your residents are and are not allowed to do. This is a perfect example of something that you may not think of until after the lease is signed and closed. It is, however, a large safety concern for many different people. It may be better to suggest your residents go to a friend's house if they want to have a backyard cookout over the summer months.