User input can be gathered when workflow is initially started or while it is ruining by using UserInteraction item.
Unfortunately vRO presentations does not dynamically adding of additional input fields based on value provided in another field. Workflow presentation need to be defined at design time with required number of input fields. Still you can dynamically show/hide some filed based on the value of another field. This is achieved by using Show/Hide input field constrains defined trough Presentation tab in vRO smart client. And it can be used to mimic dynamic number of inputs. For example you can create vRO presentation having 10 input fields where user can provide required group names and one additional where user provides number of required groups. Then you can hide unused fields based on another value provided in input box containing required number of groups.
Second option is to use UserInteraction activity which will suspend vRO workflow execution until user provides required presentation. This step can be reeated as many times as required.
But as a whole the idea of vRO is to be automation tool and it is better if all required information is gathered before starting the workflow and provided to vRO workflow on start this way workflow will not pause indefinitely waiting for user inputs.
This sounds like an unnecessary complication to me.
Instead of asking for the number of groups, why not just have an input field of type Array/AD:UserGroup where the user will select all the groups he wants to put the user into?