If you remember my opening story, you'll know this one's personal for me. My boss who slipped away when the bill showed up? Well, the rest of us had to sit there figuring out how to split the tab about eight different ways and include his portion of the bill, too. It was uncomfortable and unfair, and it often degenerated into an argument.
Your staff shouldn't have to pick up your tab. In this situation, you're actually there to take care of them. If there's a group outing, a lunch meeting, or an office party, the boss or the company pays the bill. Not only is this standard etiquette, but rarely do employees make as much as the boss, so give us a break!
Think of picking up the check as another opportunity to show your leadership. Don't leave your staff in the awkward position of having to discuss personal finances after a lunch meant to discuss teamwork. If for some reason you or the company is not paying for the outing, make that clear before the group leaves the office — and make attendance optional, to be fair. But even better, have a fund set aside for these occasions. Or go with potluck.