AvoidingDM, it depends.
There was a child who joined DS' reception year in February, it really made me think.
My DS' experience (starting at just 4, with everybody else, FT after one week of half-class settling in mornings) was not an experience of an 'easy settling in period' - there were 28 other children needing 'settling in', my DS who was very anxious but quiet and well behaved was totally overlooked, leading to considerable difficulties. School life was not so much explained to him as just assumed that he'd 'get it' by osmosis. He coped by copying other children (who were just as clueless as him) and by taking any explicit instructions literally (e.g. 'Children, go to the loo if you need to, we'll have carpet time next and I don't want you to be going to the loo then'. DS doesn't need toilet so doesn't go. Comes carpet time, needs a wee. Has 'been told' he can't go to loo during carpet time, so wets himself instead.)
The child who started in February (at age 5) had a lot of individual attention, everything was explained to him rather than assuming that he'd just 'get it' along with everyone else. He could do very well by copying the other children who were settled and knew what's what. He was assigned buddies to help him out when he was unsure. His settling in period was a lot gentler and individualised to his actual needs than my DS had.
And regarding friendship groups. One of the main issues DS had was that for the first half year of reception or so, the children were establishing a social hierarchy (not quite the same as friendships!) As a four year old he was socially confident (for his age), friendly and outgoing, but unequipped for dealing with the kind of manipulations, boundary drawing, in-group creation and explicit exclusions that were happening. It was only after that phase had settled down (hierarchy established) that friendships started to emerge. I wish I had spared DS that experience - it made him grow up before his time, making him socially quite anxious now.
The child who joined in February joined just at the right time when children had got over the 'hierarchy' phase and were starting to form friendships.
But I recognise that not every child's experience of the beginning of reception will be like this (or else I would DEFINITELY be delaying DD's start!) I just think it can happen, and average 4yos are not equipped to deal with this. So the arguments about 'easy settling in period' and about 'establishing friendship groups' don't always apply, IMO.