Years ago I met A and then met B, we became a three, B then introduced us to C and D.
Over the years we have done things A,B&Me, C,D&Me, B,CandMe, Everyone all together or a mix of at least 2 of us up until a big group of 5.
We all have a favourite restaurant and sometimes it will be 2 of us, sometimes all 5 of us. Some of us live closer to others so can pop by more off hand, others have more space for guests, or have the capacity to have someone come to stay (last year I went to stay with D for a few days)
I have autism and often feel social norms aren't outwardly expressed but the above is friendship.
When I had a rough time - I didn't want to impose but one friend invited me to stay, when another friend was having a run of ill health, I popped in to see her.
We have separate friendships that evolve and sometimes a text between 2 of us leads to an activity just us - ie finishing work early and grabbing a drink before the train, and sometimes we see another has gone to stay for a weekend with another. But harmoniously as a group we mix and interact our own ways and relationships and are better for it.
maybe invite the one thats having a rough time out for a coffee the 2 of you - this is how deeper friendships happen.