You can give each Role additional permission to point users to other roles by unchecking the "Can assign employees to roles in the Process" switch when creating (or editing) the Role.
Leaving this permission on, the created Role, e.g. HRBP, will be able to select users in its Portal to all other roles that participate in a given Process.
To narrow this permission and give, for example, a Manager the ability to select only a Buddy, you can select below the roles to which a Role can point users.