The currently accepted answer depends on automated passport control (APC), but not all airports have APC.
I have traveled to the US with non-US-citizen companions on many occasions, beginning around 15 years ago. I've never used APC.
CBP officers have told me on several occasions to bring my non-US companions with me in the US citizens' line. More ...
This depends on the arrival airport, and whether your friend have visited the USA after 2008.
If you fly into one of the airports which has Automated Passport Control (the link lists all them and it seems like every major airport is there), then YES, your friend can use this line if he visited the USA after 2008:
Who Is Eligible to Use APC?
U.S. and ...
Who gets these is a closely guarded secret but the answers and comments on Can others in my party go through TSA PreCheck? definitely suggest having someone with a KTN makes Pre Check happen for the others. Choice quotes:
it seems that there is a higher likelihood of the TSA pre going to some traveling on the same record as another KTN holder.
Yes, that's ...
The main problem is these days, almost every big site tries to bill themselves as a community. In addition, many of them don't release stats on demographics, membership numbers and so on.
Some of the most popular ones include:
Couchsurfing - over 5.5 million members in 97,000 towns, in over 250 states or countries.
TripAdvisor claims to be the largest ...
There are quite a few sites aimed at bicycle tourers. For example:
http://www.crazyguyonabike.com/classifieds/ (the whole site is good, this is just where people post ads for companions).
To use the WIZZ Discount Club, your girlfriend's ticket must be on the same booking as yours. Unfortunately you cannot add passengers to your booking, so you need to create a new one. But I assume that it will cost more so it's not worth it.
Can I add more passengers to my reservation:
Unfortunately this is not possible. You will ...
I am not sure whether this is what you're looking for but couchsurfing may be a starting point:
as they claim on their website:
Couchsurfing is a global community of 14 million people in more than 200,000 cities who share their life, their world, their journey. Couchsurfing connects travelers with a global network of people ...
Well you could try hanging around in the pre-customs area for your friend, but you'd need to know the gate, and that it's the same terminal (they have more than one), and be prepared to be ushered along if someone spots you loitering. One solution is that you could go into the bathrooms that most airports have by the gates and freshen up, using up some of ...
I found one that might work, literally only on the past weekend. So good timing, potentially. And I'm aware of a more adventurous companion one too.
Kicktraveler - the American spelling of traveler, but I'll let that pass - allows you to join or design trips that you want someone to come along with. It's like crowdfunding your travels - say you want to ...
The American Express Platinum Card has a international companion airfare benefit, however it requires you to purchase a business class or first class ticket.
Alaska Airlines also has a credit card featuring a coach companion airfare benefit: Alaska Airlines Card
I second meetup.com as a viable source though the quality of what you find in each city varies a lot.
There is also internations.org which is an expat organization which usually has groups in most major cities around the world. But you might need to become a paying member to access the activity groups.
And then there is a digital nomad group nomadlist.com ...