There is no real default gender for groups of people, you're free to use either the masculine or the feminine; people tend to use whichever there is more of in the group, and if they don't know they tend to use their own gender (so women would normally describe an indeterminate group as a feminine plural, and men would normally describe an indeterminate group as a masculine plural). As there isn't a gender-neutral (ntbcw neuter) third person pronoun (although you can use the impersonal form in some contexts), the same principle applies for describing a generic person: Women mostly default to feminine, men mostly default to masculine. Furthermore, the neuter is used for the plural when you're talking about groups of differently grammatically gendered objects and animals. When they're in an agent/subject position, some people "anthropomorphise" them by using the masculine or feminine instead as they would for people, but to some speakers that sounds strange, so the learner would be advised to stick to the neuter.