You can create personal groups on the User List in the "Favorite users" folder. To these groups you can add the users to whom you send messages most frequently. Using this feature you can do it with two mouse clicks.
First, you should specify a group name ("Add new Group..." button). When done, select the group and add users to it. Users are always added to the group currently selected in the User List. If you want to add users not available in network at the moment, you can type their names in manually. It is possible to add multiple names, separated by commas.
After that you can then organize "Favorite Users" groups directly from the program main window via User Context Menu.
Attention: Don't create groups with the names identical to the names of users or other objects existing on the network.