There is no "only-child personality" it's a total myth.
My DS is an only and has over the years been along to lots of clubs, activities, holiday playschemes etc and been more than happy to go in on his own without knowing a soul there - it's in his nature to be like that because he's naturally not phased by very much, isn't a worrier, is very easy going, socialable, extrovert and makes friends easily. It must be in the genes to a certain extent because I'm also very out-going and sociable. He was the same with starting school and pre-school etc. He never worried where I was.
By the same token over the years I've seen loads of chidlren at these various clubs and activities and in the school playground and at birthday parties etc screaming and crying and clinging to their parents and having to literally be peeled off them to get them to do anything! In the two very worst cases I can think of seeing over the years the children in question both had a sibling with the standard 2 year gap. One girl was so painfully shy that she used to miss whole birthday parties because she flatly refused to get off her mother's lap and take her head out of her hands, and the other boy used to suffer from extreme separation anxiety and still hadnt settled into school properly by the end of Yr 2.
How a child turns out will be individual to that particular child. it wil be determied by whats in their genes and how they are brought up by their parents.
I think parents of only children are sometimes guilty of "buying into" this whole "ony-child" personality myth and look for a convenient label to hang their child's clingy or shy and retiring behaviour onto when in fact it's just their particular personality.
It's not always practical to take a sibling along to everything they do anyway - lots of things have a specific age limit (such as football teams, or cubs etc) or are not things that oppostite sexes would be intersted in.
We can't have a comfort blanket throughout life so I think it's a good skill to nurture in a child to learn to face challenging situations. Doing it will give them sese of achievemet and help them to grow as individuals.
Can a school friend not start an activity with your DC so that they've got some company and someone they know with them who will be in the same boat?
It's my expereince of children with sibligs (both within my family and friends families) that they often spend enough time with them and want to make friends and spend time with other children rather than their siblings all the time.