Can I use different currencies?

Yes. Each group has a group currency for settling debts. You choose a display currency under Language & region: balances and totals are shown in that currency. Expenses keep the currency they were entered in; payments are recorded in that same currency. Exchange rates are only used for display totals and show when they were last updated.

Going on a trip? Read Split travel costs with friends, with a worked example for 4 people.