So Twitter and Friendfeed dont support groups, doesn't mean the client app cant! :)

I want the ability to create groups, and assign people I follow to those groups, across both Friendfeed and Twitter contacts.

I kinda imagine this as being able to right click on a Tweet/update or the username of the person the tweet/update came from, then it saying something like "assign to group" with a foldout of the groups I made to choose from.
Alertthingy would then always associate that users Tweets/Updates with that group.

Every user would have an associated group name somewhere nearby, perhaps in small letters underneath the username.
You can then choose different views to watch. the general view would be the default, and show all updates, and you could "tab" easily between groups.

This would give us all some level of control over all the stuff we follow. And this can extend to any other service AlertThingy chooses to support.