Having spoken with several people about this, both inside and outside Answers, I think we all agree that establishing variables in an "Invoke Business Process" activity is awkward and inconvenient at best, and one step short of horrifying at worst. 

For large forms with many variables, you have to either set up multiple 'Invoke' activities with different manually-selected subsets of variables, or use the dreaded scroll arrow to... slowly... add... every... variable... every... time.

If you need a workaround, there are some good suggestions in my related question post here: https://answers.laserfiche.com/questions/211229/How-do-I-get-all-fields-in-a-Form--get-table-variables-via-Javascript

Now, I will say, I am glad that Laserfiche has created this activity so that it is an option.  However, it would make FAR more sense and be FAR easier to use if we could simply add all selected variables, or, better yet, use a modal similar to the Assign Field Values or Fill Out PDF Form activities.