You were invited for a team event and you should go unless you are not available for official or personal reasons during that time.
None of the reason can be "I do not know the person".
If you do not know him, this a chance for you to get to know him along with several other things about the company, culture and people.
workplace.stackexchange.com/users/66920/PagMax
Absolutely.
While there's one person going to this lunch who isn't going to be around very long, there are going to be a lot of other people who are going to be important to know and work with in your job, and it's a good way to meet people.
You're likely to not be able to make the best of the networking opportunity, but even just having people know that you're the kind of person who goes to these sorts of gatherings will probably help your career, even though many of these people won't actually get to know you.
workplace.stackexchange.com/users/100444/Ed Grimm
Free food.
End of story.
Also, a chance to meet the rest of the team in an informal setting.
workplace.stackexchange.com/users/27019/Ed Heal
This just sounds like a great opportunity to get to know your whole team; not just to wish someone farewell.
Farewells are usually much more laid back than a typical lunch and you will be surprised at the stories that you get to hear.
Hiding behind the guise of "I don't know them" will paint a picture of you that you do not want to be painted.
workplace.stackexchange.com/users/17532/MonkeyZeus
Personally I think at the end of the day it's entirely up to you.
Don't feel obligated to go (you're not).
Don't feel like you're not wanted either
(you are!).
Some folks go to meet people or network or get a free drink or bite, other's don't go because they can't or don't want to
(it can potentially be and feel quite awkward if you're not very social).
It's all perfectly fine for a casual farewell lunch.
There will likely be more of them and sooner or later you'll likely find yourself going to hang out with some of the folks you know one way or another.
So don't sweat it.
Unless for some odd reason your manager tells you that you should be there, then make an effort.
But that'd be unusual in my experience.. Not that it matters but for what it's worth I've been working at all levels for 30 years.
Currently in senior management.
workplace.stackexchange.com/users/106212/Gerald
I don't know the correct etiquette to follow for whether I should accept or decline the invite, since I don't know the person at all.
You don't need to know the person, and in fact, this is your opportunity to meet them.
The invitation was sent to the team, you are a member of the team, unless you have a pressing personal or professional matter that would prevent you from going you should go.
workplace.stackexchange.com/users/60072/joeqwerty
It's understandable.
But this is a team lunch, not just a few guys hitting the bar after work informally.
I would accept it, and then perhaps express any concerns to my manager.
If you're concerned, ask if it's appropriate.
More than likely
he/she will say just as many already have that this is a teambuilding thing, and it's good for everyone to be there.
workplace.stackexchange.com/users/100951/Keith
Probably the correct thing to do is to go, but what the other answers are missing is that it depends on the culture of your country/industry/workplace.
If it's a team lunch, being paid for by the company, then almost certainly you should go.
As other people have said, it's a good networking event with your colleagues who are staying.
One of the other alternatives is that the person who's leaving is buying everyone a round of drinks.
(This is common in a couple of companies I've worked for.)
Although they probably wouldn't mind buying you a drink, I guarantee it will make you feel awkward!
(Whether you get included on the round, or refuse and say you'll get your own, it's going to be socially awkward for you.
Some people wouldn't find it awkward, but by the very fact you've asked this question you're clearly not one of them.)
So, how to find out?
Ask someone you trust at the company.
This might be someone who was assigned to settle you in (show you where the kitchen is, tell you how to work the IT system), it might be your colleague who sits next to you, or it might be your manager.
If you ask them "Should I go to this event?
I don't know the person leaving
and I don't want to put them out at their celebration", you'll find out very quickly what the culture is like at your company.
And they won't be surprised by the question either, it's a very natural question to ask.
workplace.stackexchange.com/users/33677/AndyT
Go, so that you can get to know the person.
By doing so, you will end up making a connection.
The bigger your network connection, the better.
Besides, you never know, you might just run into him one day.
He might even help you get a job one day.
workplace.stackexchange.com/users/86457/Mantie Reid II
In this company, when someone leaves the secretary collects money for a leaving present.
Then you sign a card as well.
So go to the secretary (or whoever is responsible for this), cough up your share and sign.
Then you will be entitled to turn up.
Actually, since you are new no-one would bat an eyelid if you just came anyway.
Just don't eat the whole buffet.
workplace.stackexchange.com/users/14290/RedSonja
If it’s free lunch yes!otherwise just send a polite farewell by email.
workplace.stackexchange.com/users/106295/aaaaa
I just started a new job and I was sent an invite to a farewell lunch for a coworker on my team. I don't know the person and haven't talked to/seen him yet. The invite was sent through an email with everyone in the team on it and NOT specifically to me.  I don't know the correct etiquette to follow for whether I should accept or decline the invite, since I don't know the person at all.
I just started; should I accept a farewell lunch for a coworker I don't know?
workplace.stackexchange.com
<new-job><lunch><farewell>
workplace.stackexchange.com/questions/139120
workplace.stackexchange.com/users/47500/a wee little worker
It would generally be seen as appropriate to attend, but it depends on the specific culture in your workplace. At the end of the day, it's up to you to decide if you want to go or not. The appropriate answer to this question may well depend on the culture within your company, industry or country. However, whilst you're not obliged to go, most people would probably recommend attending, if only to meet colleagues and broaden your professional network.
Most people would probably say that you should attend, if only to get to know other people that you work with a build your professional network. You are not obliged to go and your decision will be entirely up to you. The answer to this question may well depend on the culture in your company, industry or even country.