The usual rule in French is called “Le masculin l’emporte”. It means that when you have both genders in a phrase, the masculin takes precedence.

It also works for adjectives and verbs:

Marie et Jean sont allés à la cantine.

Le camion et la voiture sont verts.

It becomes ils.

According to Wikipedia’s article on French personal pronouns:

In French, a group containing at least one male or one masculine noun is considered masculine, and takes the pronoun ils. Only exclusively female or feminine groups take elles.

Aside: When I was learning French, I wanted to test this rule and pestered the teacher with questions like “What if there are 99 women and one man?” Since then I’ve found that this is a common question. In real life such extreme cases probably depend on how formal you’re being. But the officially correct answer in all such cases is ils.

You should use Ils as explained by Luke and Gilles. But, we see more and more sentences like:

Nos collègues sont invité.e.s à participer sous réserve de l’accord de
leur supérieur.e hiérarchique.

Where invité.e.s is a way to express the importance you give to the female people: collègues can represent men and women but the author deliberately uses a syntax that is supposed to represent both using the dots.

In the same way leur supérieur.e hiérarchique is a way to say that your superior can be a man or a woman. NB: in this case, a more classical way could have been:

de leur supérieur (ou supérieure) hiérarchique.

PS: I know this is not an answer and should have been published as a remark but the number of characters is too small and the rendering too poor. Don’t vote for this answer. Thanks on behalf of women 😉

I learnt this the other day in my GCSE lesson,

Say you have a group of 100 GIRLS then they are referred to as ‘ELLES’. All it takes is for 1 BOY to be added into the group and now the group is referred to as ‘ILS’.

If you are comparing a girl to a boy, for example, Sophie is more tall then Billy, the french for taller would be the female way. so it will be: Sophie est plus grande que Billy.
However, if you are comparing Billy to Sophie then the ‘GRANDE’ will change to ‘GRAND’:
Billy est plus grand que Sophie.