Place the sausages in the bowl of a slow cooker. Add the barbecue sauce, brown sugar, vinegar, and beer and stir to combine.
Cook on high for 2 to 3 hours or until the sausages are warmed through.
Lower heat and keep warm to serve.
Notes
To thicken the sauce: Stir in a cornstarch slurry (1 tablespoon cornstarch + 1 tablespoon water) and cook until thickened to your liking. Storage: These will keep in the fridge for about 5 days. You could also store them in an airtight container in the freezer for about 3 months.