Unwritten Social Rules You Should Know To Follow

When invited out to dinner, and the host is paying, watch for what he or she orders. Order something the same price, or lower. The host’s chosen price is the indicator of what he or she can comfortably pay for everyone else.
The most important thing is, if you are a guest, know this rule, so you have some guideline to follow if nothing else is said.
A buffet is a simple solution a host might choose. The price per person is set, and each guest can enjoy what they like. Fun
This whole method came about in the 1950s or earlier, when it was much more polite not to be explicit about money. These days, if you can, you can say “I’m paying, but if you want something over $25, please chip in.” That way, you can order whatever you want, and everyone can know the guidelines.
Obviously, this old etiquette rule has its downsides. Say everyone knows the rule. The host must order food at the price he can afford for everyone, even if he would prefer something cheaper - like maybe he likes grilled cheese or is a vegetarian or is eating only salad to lose weight.
The host or hostess can inform others by saying “I’m having the X dish” before the waiter comes around to take orders. Or the host can arrange to order first, and speak loudly so everyone hears.
Of course, if the host does not say what he or she is ordering, you can’t use this rule. See several options in the comments, such as simply ordering something reasonable, or not ordering first and matching price or item for what someone else does.
As we do get more up front about money and offers to take people out, there are options like ordering something cheaper, but telling people to go ahead and order what they like, or order up to a certain amount.
Alcohol complicates the issue. You may want to let people know that they can pay for their own, or set a one-drink limit on your own tab.
Groups that go out regularly often have their own rules. That works very well. See some excellent options in the comments