I've never been to an autism group offline, but I'd be intrigued to know what it would be like.
I'd probably be shy at first, then once I get to know people I start talking more. Autistic people are still just as human as NTs, so the same social anxiety applies with me.
When I was a teenager I went to a club thing for teenagers with disabilities, but I felt I didn't really fit in there. My mum wanted me to join so that I could make friends, but it didn't really work out. It felt a bit childish there, like the karaoke was of kid's songs and they organised Christmas discos and played board games that were suited to younger children.
I think I was the only one there that went to mainstream school. A lot of them were quite low-functioning autistic and a lot weren't autistic but had other things that impaired their social and intellectual functioning.
Then when I went to college I found myself in a group of other teenagers that were more like me, like people with Dyspraxia or ADHD or Asperger's/high-functioning autism. We all seemed NT but just with quirks and some challenges in things like reading, writing, math and finding employment, and the course provided some short drama classes to help with social skills for adulthood. It was aimed at young people who got low grades in their school examinations and just needed that bit of extra help preparing for adult life.
I made friends there, although some were fostered adolescents so were a bit troubled, while others were shy and lacked confidence.
I made friends with a girl who had Fragile-X syndrome, which is quite similar to autism.
People with FXS can be shy, have difficulty making friends, can have sensory issues, and suffer with anxiety and depression, without necessarily having autism with it. Some have intellectual disabilities with it too, although this girl didn't really. She had the same sort of intelligence level as me, which I call "surface level intelligence". I should start a separate thread on that.