Help with adding RoleDefinition to a group

Hi,

I'm quite new to SharePoint development so this is most likely a simple question. I want to add a permission to a group and I can't figure out how. I've retrieved a Group object and a RoleDefinition object, now I need to combine the 2.

I've found out that in SharePoint 2010 I could use this call:

But I'm using SharePoint Online and this call is not available for me (It need a principal and not a Group).

Can someone point me in the right direction?

Thanks in advance!