Assign Contacts to your Tasks and Board to identify the combined team including your internal members and Contacts that could be from a Customer or a Partner. Contacts can be assigned as Board Team Members with a Role and assigned to specific Tasks. When a Board is shared through External Sharing everyone can see who has been assigned and to which task.
How to add a Contact Role to a Task or Board
Assigning a Contact works the same as assigning a colleague. From the Board Team member screen choose "User" or "Contact".

How to Set Up Contact Roles
Add Roles to the Picklist Global Value Set “Team Role”
- Navigate to Setup > Picklist Value Sets > Team Roles
- Add Roles that will be used

Assign Roles to Record Types for Board Team Member and Task Contributor
- Navigate to Setup > Object Manager > Board Team Members
- Select, Record Type, then for both the Internal and External Contact Record Types:
- Select Edit alongside the Role Picklist
- Then Add or Remove the Roles as appropriate to define them as valid as Internal or External Contact Roles.
- Select Save
- Repeat this step for the Task Contributor object ensuring you maintain the same option between each.
Note: If roles are not assigned to the Record Types then the Role will not be available to select.

Assign Internal Users and External Contacts to Boards
When you have your Roles and the Internal and External Contact Record Types defined you can choose to add Users and Contacts to your Tasks and Board.

Assigning Contact Roles on Task Templates
When you have your Contact Roles defined you can include them in your Task Templates by adding a column labelled "Contact Roles". You can relate multiple Contact Role to the same task by separating them with a semicolon (;).