There must be a ruleset to fill ManyToMany fields.
Example: adding users to a group.
You don't have the option to define the relationship in the Delete relationship command but you can select from an existing list, relevant to the record you are on,
This means you can run a rule to delete a group membership relationship, either from the Person or the Group workflow.
In my example below, I decided to perform the deletion from the Person workflow. Since you will likely initiate the task from an offering, you need a way to trigger the workflow. I added a custom field on the Person workflow called GroupToDelete_c, which is an ENTITY_LINK to the Group record type.
In the offering, the task is to set the GroupToDelete_c field for the Person record impacted. You can have the same two user options I showed you in the "add membership" offering.
The Offering task plan will have an automatic task to update a field on another record:
Then on the Person workflow, you will have the rule to delete the relationship
The inconvenience with this approach is the need to use a custom field on the Person record, but, assuming you don't need too many custom fields on the Person record, it is a reasonable compromise. You don't even need to add the field on the Person forms.