Will I get charged for the people I invite to multi-company groups on Workplace?
No, you won 't be charged. If you invite someone to a multi-company group (MCG) whose company isn't yet using Workplace, they'll be set up with a free Workplace Essential account. If they're already using Workplace Essential, there will be no change to their status. In either case, there will be no applicable charges.
If the person you invite to an MCG works at a company that's already using Workplace Advanced, you similarly won't be charged.