¿Ele or Ei?
Ele is feminin, so if it was a group of all woman it would be Ele, but If it's a group with at least one man it's Ei, this doesn't really tell you so technically it could be either.
Why is it sunt in this case?
Because that is the form you use with this verb.
El/ea este (e)
Ei/ele usually shares the verb form with either el/ea or eu.