There wouldn't be a way in which to do this without someone having to approve them. However, I do have an alternative idea for you.
Have you thought about using commerce perhaps? This way you can use forums as you intended in the first place, rather than clubs. So using your scenario above you could create either a subscription or a product, with $0 value with the name "Microsoft user group". Create a group that is added as a secondary group when a user "purchases" that item, which also gives access to x forums content.