Since these are your friends just ask one of them, or the group, if they can loan you the money for lunch. ThisThis shows you are willing to pay and aren't just assuming they will pay for you but letting them know that you don't have the money right now.
- — Hey! Let's go for lunch!
- —
— Actually I don't have any cash at the moment and I brought lunch. If one of you could lend me a bit until pay day I'd be happy to go. Alternatively put off the celebration until pay day and I'll treat you guys for a change. —
Asking for short term loans of small amounts is pretty normal, even more so when it is something like this where you had a different plan that wouldn't have required a loan. ChancesChances are, based on previous experiences you related, they will tell you not to worry they are paying and the situation is resolved. NextNext most likely is one of them spots you the money and you pay them back.
My assumption here is that you don't mind spending money on lunch as a general rule, you just don't have the cash at the moment. WantingWanting to cut back on frivolous expenses (certainly I prefer not to eat lunch at a restaurant every day) iswould be a different question.