When throwing a party, the cost of providing alcohol for all your guests can become quite steep. This is especially true for larger parties, or if your friends tend to bring additional guests. With the exception of potlucks, it's generally assumed that the host will provide some kind of snacks or refreshments, but it's also acceptable to ask your guests to help by bringing their own drinks. Ideally, your invitation should make this perfectly clear.
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Write the invitation normally, indicating the time, date, and location of the party, as well as any particular reason for throwing it or special requirements such as costumes.
Write "BYOB" at the lower right-hand corner of the invitation. Make it is large enough so that your guests will see it. Don't worry about spelling it out, as the acronym is nearly universally understood to stand for "bring your own booze," "bring your own beer" or "bring your own bottle." If you're sending the invitation by email, you can simply include "BYOB" at the bottom.
Add an additional request, possibly as a postscript, for people to contribute a few dollars if they are unable to bring their own drinks. Even if you ask people to bring their own bottle, some will inevitably forget or be in too much of a hurry to do so. By asking for a monetary contribution, you can either offset the costs of whatever drinks you do purchase or provide the funds for a last-minute beer run.