Learn Italian: Le cicliste hanno letto la mappa in piazza. - The cyclists read the map in the square.

Nouns ending in -ista are unique: they have the same ending for both masculine and feminine in the singular (il ciclista, la ciclista). But in the plural, they change to match the gender: masculine becomes -isti (i ciclisti) and feminine becomes -iste (le cicliste).

Does the feminine plural le cicliste mean there are no men in this group?

Exactly. In Italian, if a group contains even one male, you must use the masculine plural (i ciclisti). Because we see le cicliste, we know it is an all-female group.

Since the subject is feminine plural (le cicliste), why doesn't the past participle change to match it (hanno lette)?

Because the helper verb is avere (hanno). In the passato prossimo (past tense), the past participle only changes its ending to match the subject if the helper verb is essere. With avere, the participle (letto) stays in its default masculine singular form.

Why is it just in piazza instead of nella piazza?