A mechitza to separate men and women is needed only when praying in a place dedicated as a permanent location for regular Jewish prayer services with a minyan.  If not, no mechitza is needed, and any separation that prevents distraction between men and women is sufficient. Even if the women are just behind the men, that is sufficient, even without a physical barrier.

Men and women should be separated during prayer services with a mechitza that blocks the men's view of the women. The goal is to keep the men and women from distracting each other during prayer. This idea originated in the Temple in Jerusalem.

For women and room divider/mechitza, see Where To Pray: Distracting People/Mechitza.