I.e. for a group of guys, you have: 彼たち、かれら
and for girls it's: 彼女たち 彼女ら
So if you have a mixed group, which one should you go with? Is there a different word?
For a group where men are majority ("majority" means they are representative or typical members), use 彼ら. Where women are majority, use 彼女たち or 彼女ら. For unspecified mass that you don't care about its internal composition, 彼ら is the default option.
If you've already mentioned a principal member of the group in the context, the most natural way is to follow his/her gender.
It doesn't produce any grammatical difficulty, because Japanese "plural" X-たち actually stands for "(an) X and its kind/company", thus doesn't imply every member shares the exactly same features that X has. (If X is a common noun, it's highly inclined to be interpreted just like English plural, but doesn't eliminate "X and others" reading, neither.)
In more casual speech we have genderless words to use: （こ／そ／あ）の人たち and （こ／そ／あ）いつら (rude).
Last but not least, have second thought before you're going to use "they" in Japanese. Pronouns only for keeping the subject's place are usually useless in Japanese. If you need to disambiguate them from others, best use the word you've previously used to call them once more.
For a group made up of both men and women, you can use "彼ら."
Today's youth, I cannot understand their psyche.
The resort places are full of Chinese men and women. They are surprisingly active and demonstrate an enormous purchasing power.
The program portrayed men and women participating in a group marriage arrangement party, they all appeared to be fully enjoying the atmosphere of the party.
Of course you can say "彼らや彼女ら"、but it sounds redundant.
I think we use the words of status and relationship with speaker than a pronoun when you call a group.
For example, 先輩たち、先生たち、高校の友達、近所の人たち、etc. And we also use a person name of the group like 田中さんたち、鈴木さんたち.
In addition 彼ら can be used irrespective of gender but if you want to make clear a group is mixed man and woman, you can say 彼、彼女ら(たち)は.