To build a community of players, variation in the games presented is essential. Just playing things with the same set of items, stages, or characters will make things old quickly. Plus, as the skill levels of the players become apparent, it will be difficult for players who aren't at the top tier to enjoy themselves without mixing things up some.
One of the easier options to include is to let players setlle their one-on-one bouts with each other. I usually do this as the end of our programming sessions with the Wii, and it works well as a way of de-stressing all of the complaints that have built up over the course of the match.
The players will probably want to play with a certain set of lives (or stock) to settle their matches. You can let them do this, but you'll also want to set a time limit for those matches (in the Options) so that people don't end up watching a cat-and-mouse game for a long time. It may also be wise to just set time limits for all the stock matches you play, not just the one-on-ones.
Keeping a list of one-on-one match requests is a good idea to have, so that everyone gets theirs in order and nobody feels slighted. It doesn't need to be fancy, just whatever kind of list you want to have available.
Team battles are another great way of keeping everyone involved. Players of one skill level can gang up against another, or the good players and the not as good players can team up against a computer player or team so as to feel victorious. Additionally, since those matches can be handicapped, it's possible to make things fairer to both sides.
At the end of each match, the summary provides various achievements/trophies and accompanying points values to each player. Almost everything that can be done in a match has a corresponding point value and achievement. It could be a great idea to not care as much about the knockouts and falls as the total amount of points earned at the end of the match or the biggest variety of achievements. Smash Brothers Brawl offers a lot of different ways of making sure that any player of any skill can have a good time and find something they can be proud of.
One of the most useful options I have found in Smash Brothers Brawl is the rotation mode. Instead of arguing over who should give up the controller to the next players that want to play, the console can be set up to take up to four players at any given match, and then change out anywhere from one to four players from the match with the next names on the list. The game can be set to rotate either the people who do worst or the people who do best in the match they're playing. The only limitation there is on rotation mode is that the list only accepts a maximum of 16 names at a time. Thankfully, most of my programs have not gotten bigger than that.