There is absolutely no requirement that a person has diabetes before they can join the forum
There is a place in the profile information which allows people all sorts of different options, including 'don't have diabetes' 'parent (of someone with diabetes)', 'carer' and 'other'. All are welcome.
I am an 'other' myself
Having said that, I think that sometimes people without strong links to diabetes can sometimes find the forum somewhat... um... too diabetes focused for their purposes. Having D adds a rather different perspective than someone who is low carbing (for instance) without D providing that extra special twist of motivation.