I wonder whether there are languages that distinguish comitative with ("I peeled the potatoes with a friend") and instrumental with ("I peeled the potatoes with a knife")—e.g. by having separate cases or separate usual prepositions.
They're the same in English (with), German (mit + dat.), French (avec), Greek (με + acc.), and Russian (с + instr.), as far as I know.
(Now that I've said it, I'm sure there are such languages; only I don't know of an example off-hand.)
Of course, it's possible to distinguish between the two by using synonyms or other word choice (e.g. together with to emphasis comitativity), but as for the basic choice?