The Moodle Gradebook does not allow for setting visibilities based on Group membership. This means that all Activities will populate the Gradebook for all students. 

In order to calculate the grades correctly, we need to drop assessments for some groupings of students and add assessments for some groupings of students.

To do this we need to create more Grade Categories.

Option 1: Two different submissions within same Grade Category

This applies where the students are submitting different types of assessments but they are contributing the same overall Category Weight: 

e.g. The face-to-face students are graded on in-class participation, which is entered via a direct entry grade item. While the online students are graded for participation in a Forum discussion. Participation for both groups is weighted at 15% of their total grade.

1. Navigate to the Gradebook Setup
2. Select "Add Category"
3. Name the Category clearly delineating by group (e.g. "In-Class Participation" or "Online Forum Participation")
4. Under the Parent Category dropdown, select the overall Parent Category (e.g. Participation)
5. Save Changes
6. Move the relevant grade items under the newly created Grade Category
7. Set the new Category Weight to 100.0
8. Save Changes
9. Navigate to the Parent Category ("Participation") and select the Edit Dropdown > Edit Settings
10. Select "Show More" under the Grade Category dropdown
11. Enter [1] into the Drop the lowest field
12. Save Changes

Why does this work?

The Parent Category "Participation" is still worth 15% of the Total Grade. But when it calculates the weighted mean of grades beneath it, it only looks at the two Category Weights of "In-Class Participation" and "Online Forum Participation". 

Each of these are worth 100% of 15 so together would each be 50% of the grade category.

But the calculation also drops the lowest grade - which will be a zero - leaving only the single category remaining, still worth 100% of the parent grade.