If I am unsure about whether to go to something, I remind myself that it will cost me a couple of hours, and that if I didn't go I would be doing X. Usually the X is something I could do at any time (eg reading a book) and the event would mean meeting people who could become friends - filling my social bucket, as you say. Nine times out of ten I go. Also, if everyone stays at home events stop happening, and meeting people becomes difficult.
As we get older it can be difficult to make friends, as people often spend time with family, and don't go out as much as when they were younger. Also, even if we have a lot of people around us, they can move away, or leave our lives for other reasons, and as our social circle dwindles it can be difficult to fill that bucket, and many older people end up feeling lonely.
As often as not, something like a group lunch won't result in meeting a lifelong friend (although you never know), but it will mean that when you next bump into the people you chat to you can say hello, or sit next to them at a meeting and pick up where you left off. You could then suggest going for coffee, and see how it goes. The more people you have who you can call and suggest doing something the less likely you are to get lonely.