Learn Russian: После собрания остались одни женщины.

While одни can mean "alone" or act like "some", when used right before a noun like женщины, it acts as a restrictive particle. It translates to "only" or "nothing but". It emphasizes that the remaining group consisted exclusively of women, with no one else mixed in.

Could I just use только instead of одни here?

Yes, you could say остались только женщины, and the core meaning is exactly the same. However, using одни with a plural noun gives a slightly more descriptive flavor, emphasizing that the group was entirely uniform.

Why does одни have a plural ending if it translates to "only"?

Even though одни acts like the adverb "only" in its meaning here, grammatically it is still a modifier attached to женщины. Because женщины is the subject of the sentence in the nominative plural, одни must also be in the nominative plural form to match it.

Why is the word for meeting, собрание, written as собрания?

The preposition после (after) always requires the noun that follows it to be in the genitive case. Since собрание is a neuter noun ending in -е, its genitive form changes the ending to -я, giving us собрания.