Only three turned up last night at club night. We usually play two against one for three games, then rotate round so that everyone gets a go at being the single. Of course the lone person plays in the singles court with the doubles court their target against the other two, and everyone gets to serve once. It's not to everyone's taste as neither situation is ideal: the singles player is not constrained to a singles court target and the doubles players are constrained to a singles court target. I find it OK to practise serve and volley when one of the doubles. We sometimes have variations on this, such as for example the non-serving doubles player is not allowed to intercept at the net on the return unless of course the ball comes to them.
An alternative is to play singles, best of three, or three games, and rotate. I personally don't like that so much , especially in winter, when it can get cold.
We also have a pattern for when five turn up, either a singles game and a threesome as described, or more usually a doubles with one sitting out for one game and a continual rotation of players. This works fine for 30 mins or so when we need to break the pattern of usual couples, servers, returners, that has developed. We have not found the easy answer to that except for someone sitting out twice or reversing the direction. Again we have variations for this pattern, for example, a sudden death point on the second deuce in a game, so that the player sitting out in not left waiting interminably on long deuce games.
I have noticed that coaches have developed different strategies for dealing with groups of odd numbers, examples are king of the court, or the game 'winner', or they join in to even the numbers. It would be interesting to know how other clubs deal with odd numbers of people turning up. Maybe people just sit out?