When you invite a couple to dinner, it is generally considered good etiquette to pick up the tab as the host. Inviting someone to dinner usually implies that you are extending your hospitality and treating them to the meal. As the host, it is your responsibility to cover the cost of the dinner.
It is considered impolite to suggest that your guests should leave the tip. Leaving a tip is part of the overall cost of dining out, and as the host, you should handle it discreetly without drawing attention to it. When the bill arrives, simply take care of the entire payment, including the tip, without mentioning it. Doing so will make your guests feel more comfortable and appreciated.
If your guests offer to contribute or split the bill, it's a kind gesture on their part, but you can politely decline their offer and reiterate that you are happy to host and cover the expenses. The idea is to make your guests feel welcome and taken care of during their time with you.