The official answer would be treasurer.
From American Heritage:

One who has charge of funds or revenues, especially the chief financial officer of a government, corporation, or association.

That said, general usage is only when this is in an official capacity. So you are the treasurer for a club or society, and there may be an official bank account associated with this.

On a ship or a plane, the word purser could also be used:

The officer in charge of money matters on board a ship or commercial aircraft.

Both treasurer and purser are somewhat official sounding, and would sound playful at best a group of friends paying for a meal through a delegate. In your example, I might just say "John is paying for now and we'll pay him back later" or "John is handling the money/cash, so pass your cash to him".

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Alternatively, as suggested by @ChrisH in the comments, you might say "John is looking after the kitty". A kitty is defined as (Collins via TFD:

any shared fund of money, etc

A kitty, though, is usually decided before the meal/drinks/event. It's usually used as an alternative to buying "rounds" of drinks. Everyone contributes a specific amount up front, so the bill is self-limiting. Left-over (or even negative!) kitty can be shared equally among participants.

The Bank

Within my group of friends, we call this role The bank. It would be easily understood by those who have never used, nor heard, the term in this context.

John, would you be a dear and be the bank for today's lunch? We'll all settle up with you by the end of the day. Thanks!

The term comes from the traditional role of a bank as a primary lender of (other people's) money. With the modern credit-based society, we sometimes lose sight of the fact that a bank is somewhere behind each of our credit cards and accounts.

From Merriam-Webster:

bank

noun (2) 

Definition of bank (Entry 3 of 5) 

1  a : an establishment for the custody, loan, exchange, or issue of money, 

       for the extension of credit, and for facilitating the transmission of funds

The word that springs to mind for me is "treasurer". The definition of treasurer in the Oxford English Dictionary is:

treasurer - A person appointed to administer or manage the financial assets and liabilities of a society, company, local authority, or other body.

Most societies -for example, a University sports club - will have a treasurer that looks after the finances of the group.

However, it may be a little formal for a group of friends going on holiday or going to a restaurant.

If the payor settles with other members after paying then I would say he is the "funder".

Otherwise, I believe all the terms offered so far sound somewhat for a permanent role. I would just maybe offer for consideration "escrow agent" or "trustee" or just "agent" because it sounds more transactional, a non-permanent role.

It's not a noun, but someone who does this "fronts" the expenses.

7 : to give (someone) the money, material, etc. needed to do something
She fronted them a loan to get the start-up going.

(Merriam-Webster)

You could say:

You don't need to pay money now; John fronted the cash/fees/whatever. We will all reimburse him later.

You could nounify it, but that's not really grammatical to me.

You don't need to pay money now; John was the fronter. We will all reimburse him later.

In the UK and where drinks are concerned, you might also refer to "the whip" and someone "holding the whip" - at least in my experience this is more common than "the kitty" or "the bank".

A closely related term is Sponsor, an entity responsible for a group, which generally includes paying their expenses.

Dictionary.com:

1. a person who vouches or is responsible for a person or thing.


2. [...]


3. a person who makes a pledge or promise on behalf of another.

Merriam Webster:

1. [...]


2. one who assumes responsibility for some other person or thing


3. a person or an organization that pays for or plans and carries out a project or activity

English does not have a direct term that corresponds exactly with the idea of someone to whom others in the group reimburse payment. Treasurer and trustee are close, because they handle or manage money or resources on behalf of others, but those terms carry a more formal and official tone than I feel is expressed in the question.

Kitty, pot, or sometimes hat refer to the collected money itself.