In the fixed phrase "..., present company excepted" (used when making some criticism of a group that may include the people you are talking to) the work "company" is treated as uncountable, or rather it is an uncountable noun, singular in form, but referring to a plurality of people.
It is used as a fixed phrase, and you would not normally use "present company" in other grammatical contexts. Here it is used in reference to the idiom: "present company" is treated as a compound noun that means "those people who were around"
She warned those present that ...
She warned her guests ...
or allow context to fill in the object:
She warned that this could easily turn into a lecture
(Implicitly she is speaking to the people she is warning)