If there is a group of people where there are only men, they should be referred to as "ellos". If there are only women, they should be referred to as "ellas", but, if there is a group of women and there is one man in the same group, even if the rest of the group includes 50 women, they should be referred to as "ellos". This is because the default gender of a mixed group (where there is more than one gender) is always masculine. You can always say "ellas y él", to include both and keep the respective genders if you want.