You can sort players in the Lobby view by clicking the appropriate column header. Every column can be sorted in ascending or descending order; to switch the order, just click the column header again.
When you click a new column, GridGammon will remember the prior sort criteria, so for example you can click the Ratings column to sort by ratings. If you then click the Status column, players will be sorted by status as the primary sort order, and within the same status (e.g. "Ready"), will sort by ratings.
GridGammon stores both the primary and secondary sort criteria in your Preferences file and will use them the next time you launch GridGammon.
In addition, you can indicate that you always want your friends to be listed at the top of the player list, regardless of which column is selected. You can turn this on or off by opening the Window Settings sheet for the Lobby Window, and check or uncheck the "Show friends at top" checkbox.