When you have a large wedding party, it can sometimes be difficult to find a decent location to take photos. The biggest issue your photographer will come across is not having enough room to step back and get the shot.
Compression... The more people we can fit into a tight frame the less the photographer will need to step back. While that may be super obvious, its not always easy to pose 18 or more people in small spaces. The best way to break this gap is to have a few people sitting, so find some chars and everything will turn out swell.
If you have no chairs it might ruin the simplicity of the shot but you could just have everyone super close together as well.
The last option being the obvious, finding more space to take the photo.