role in

[p] stands for your server prefix

Command: [p]role in <role> <role to add>
Aliases: none
Description: add a role to members of a different role
Example: .role in boosters donators
Permissions: ManageRoles

<> = required | () = optional | don't include <> or () when typing commands

The above example adds the role donators to all users with the boosters role.