My fiance and I have been together for 8 years. We have a 15 month old son and I am a stay at home Mom. My fiance just got a new job with benefits and such, so we plan on having a ceremony now so my son and I can be puton his benefits and also because we want to be legally bound to one another

So, we decided to do a vow ceremony now and have a larger party/reception in a year or two. Well, what started out as just a few friends and close family as witnesses has turned into 50 of our closest loved ones. We have advised them a million times not to bring gifts and to just dress comfortably. We are getting married in a nearby park and have decided to all go out together afterwards to a bowling alley. Since this isn't our "wedding" we thought it would be a fun and cheeky activity to do. There a pool tables, a bar, people who want to bowl can bowl. It will be fun.
The issue is, since we initially planned on it just being a few of us, we didn't plan on spending anything. We have told our friends that it is informal, just witness us saying our vows then we will go have some silly fun. My question is, do we have to pay for everyone's bowling or for drinks and such? We in no way eluded to the fact that we would do so, but I don't want people to seem surprised the night of.
Please help! Thanks!