I just come across a word teetotalers that means "A person who never drinks alcohol". What will be the opposite of this word in the same context?
Most of the time I think you want "drinker" - for example "there will be both drinkers and teetotalers at the party" or "keep in mind that not every dinner guest will be a drinker." That just means someone who sometimes drinks - after all a painter doesn't paint 24/7, a writer doesn't write 24/7 etc. If you want something opposite in the sense of "opposite end of the spectrum" then "drunkard" while old fashioned will work. Avoid "alcoholic" because some alcoholics no longer drink at all.
At my house, the phrase Straight Edge gets lots of use - a Straight Edge kid does not use alcohol, drugs, cigarettes, refrains from promiscuous sex, and more. They use "non-Edge" to refer to those who are less abstinent in some way. They also say someone "broke edge" if they drank alcohol or the like.
A teetotaler, or “person who completely abstains from alcoholic beverages”, has many opposites, ranging from social drinkers to drunkards and a thousand states between. See long list at wiktionary's Drinking Category, and synonym or related word lists at wordswarm.net (sot, toper, drunk, soak, tosspot, lush and many more, with definitions), dico.isc.cnrs.fr (alcoholic, boozer, dipsomaniac, drinker, drunk, inebriate, lush, rummy, sot, souse, tippler, wino, ...), onelook.com (sot, toper, souse, drunk, inebriate, rummy, wino, alcoholic, boozer, lush, bacchanal, dipsomaniac, soaker, tosspot, ...), et al. (The parenthesized lists after those three links are for drunkard.)
Also see the wikipedia article for teetotaler. At times during the 1800's, teetotaler referred not to just non-drinkers but more particularly to outspoken advocates of abstention from alcohol; opposites of such might be people with no opinions on the subject or of opposite opinions, a broad field. Note, the Etymology section of the article includes two possible explanations of the origin of teetotal.