I've not been a member of one of these, but this is what I gather from Internet forums and from friends who attend smaller similar churches I'm the UK:
Firstly although the Sunday services are huge, that is only a small part of the deal.
Most people belong to a small house group. Which meets on a weekday. This is your support group, where you can discuss problems, pray for each other and help each other. This is where you 'know' people and are 'known' .
Membership of these groups is sometimes geographically based, sometimes time based (eg people who can make it at 6pm on a Thurs) sometimes age or interest based, eg students, teens, mums with babies etc.
As well as that a person would probably be part of a ministry, eg food bank, homeless shelter, youth team, elderly visiting, music, etc etc. So you get to work closely with those people and get to know them as well. They will probably have a similar outlook on life to you.
Also there are specific meetings for kids, youth, young adults, students, retired folk. Early morning business-people's prayer breakfasts, parent and tots groups etc etc.
So a Sunday morning you may gravitate towards those who know you or stay anonymous.
Are the definitely therefore small friendship groups? yes. Can these sometimes get cliquey or exclusive? Yes.
But everyone should be trying to be accepting and loving towards all. Christians don't (shouldn't) think they are good people, the idea is you know you are a sinner, but you ask Jesus to help you be better.
Are some Christians crap at doing that? Yes.
But there is also a recognition that humans only have a capacity to be in a relationship with a certain number of people at a time. So no-one would expect you to have the same level of relationship with 6,000 people as you might have with 50.