The Split the bill button on the home screen is built for the situation at the table: phone out, recorded, split, phone away.
Enter the total, say how many people are at the table, and fill in the names — they are pre-filled from whoever you split with last time, and from your contacts if the browser supports it.
One tap does two things: it records an expense for the full amount from your account (the money really did leave) and creates a debt owed to you for each other person's share.
An amount that does not divide evenly is spread one cent at a time — the shares always add up to the bill exactly.
Then it is not an expense: no money left your accounts. The Debts section has a button for recording a bill somebody else paid, which creates a debt you owe for your share. It becomes an expense when you pay them back.