Bring your own is a sensible default for a lot of casual gatherings. It keeps the cost off the host, it lets people drink what they actually like, and it removes the pressure to stock a full bar for a crowd with unpredictable tastes. It can also turn into a fridge full of unlabelled bottles and someone quietly drinking the good stuff that was not theirs. A little structure keeps the good parts and loses the bad.
Say what BYO covers
BYO means different things to different people. For some it is just drinks. For others it stretches to food, a chair, a plate. Be explicit about what you are providing and what you are not. "Bring your own drinks, food is sorted" is clear. "It's BYO" on its own leaves people guessing, and someone will turn up with nothing assuming you had it covered while someone else brings a full esky. When you set up the event, the BYO details and a things to bring list do this job for you.
Provide the bits people forget
BYO falls down on the supporting cast. People bring drinks and forget ice, a bottle opener, cups, somewhere cold to put it all. As host, cover those even on a BYO night. It is a small cost and it is the difference between a smooth evening and a frantic search for an opener at eight o'clock. You are not providing the drinks, you are providing the infrastructure that makes everyone's drinks usable.
Make sharing the norm
The slightly mean version of BYO is everyone guarding their own supply. The better version is a culture of putting a bit in the shared pool. You can set this tone yourself by having some communal drinks and snacks out from the start, which signals that this is a generous night, not a transactional one. Most people match the tone the host sets. Start generous and the room follows.
Plan for leftovers and stragglers
BYO nights end with a pile of half finished bottles and abandoned cans. Decide in advance what happens to them. Tell people to take their unopened drinks home, or accept that you are inheriting a random bar for next time. Neither is wrong, but deciding avoids the end of night confusion where nobody knows whose what is what. A clear word as people leave saves you a fridge archaeology project the next morning.
Why BYO is worth it
For all its small failure modes, BYO is genuinely good for casual events. It makes hosting affordable, which means people host more often, which is the entire point. A host who does not have to fund an open bar is a host who will throw the next one too. A little coordination is a small price for keeping that door open.