Need to group many to many relationship based on their category in the form. Input: CheckboxList.

Each user has a Profile resource, which has many-to-many relationship with Skill class, which has name, category fields. In Edit Profile form I want to categorize Skills based on their categories. At the moment, all of skills are listed to together. Categorization would significantly increase readability, as platform aims to provide service to all types of professionals in the sector. I tried having multiple CheckboxList inputs, querying skills based on their category. Visually it worked, but would not save the relationship properly. Guidance in either properly grouping or saving separate CheckboxList inputs would be appreciated. Thanks for your time.