Purchased V5?? The current version is 7.0 so don't know how/why you have purchased 5.x? (from 3 years ago...?)
Yes, you need to add the users as 'administrators' so they can work in the back-end of the Machform application.
I don't know if by 3.5 whether the extra admin users was a feature, so you'd need to update to use the function. I wouldn't update to 5.x I would get 7.x. - a lot newer, more reliable with many more features.