I've used them before mainly because of the faucets but I eventually stopped after I found this forum.Afaik xapo should only charge you fees if you made a bitcoin transaction with a non xapo account. I don't think this is a good solution to save miners fee because you can't back up your bitcoin wallet on xapo, there's a chance your bitcoin could be stuck/lost once the site goes down. For group transactions I use blockchain/electrum to save fees.