It could be useful if the plasmoid could be configured to show only contacts in one group instead of all the contacts
I agree, this would indeed be useful, especially in combination with Activities. This would make one of Aaron's visions for Activities - showing only relevant contacts within an Activity - possible, without even having to explicitly associate the contacts with the Activity.
Dear user, unfortunately Telepathy is no longer maintained. Please migrate to another solution, e.g. for Jabber a possibility is Kaidan, for Matrix a candidate is NeoChat.