Easy / Awesome dark beer!

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Since you had a very active fermentation, adding more yeast would not effect the alcohol level. The sure fire way to get more alcohol is to add more sugar, in the case of beer that means grain sugars either dried or liquid malt extract.

Taking a hydrometer reading before and after the fermentation is the only way to really know the alcohol level.

If the beer goes flat you can add some sugar to the drink and let it sit at room temperature. The yeast survivors will kick in and make more fizz.