I'm in situations like this often. Is it that they want to discuss something related to work, or just hang out? If the former, I would offer an alternative scenario for meeting. I typically say I'm not a drinker but will be happy to get together to talk. Then if conditions permit, let them choose the circumstances, or I offer something (eg. a coffee or lunch meeting another day). If it's just for hanging out, I think a simple "thank you for thinking about me but I prefer not to go out tonight" would most likely suffice. If they insist, I cut that line of conversation short. No need to excuse yourself at all.