Most receptions begin with the formal entrance of the bridal party, with the band leader or maitre'd announcing each couple's name just before they enter. It's similar to stars being introduced at a gala and the process is accompanied by fanfare and applause guests. Find out where the group enter and then set up your equipment to ensure a good camera angle.
It is not always necessary to photograph each couple as they enter, especially if you already photographed them in the procession down the aisle at the Church or when you did your environmental portraits. Photographs of the bride and groom, however, are another matter. Take a shot when they first enter (with the guests in the corner of the frame) and include the guests cheering and applauding them if possible.
Some entourages line up in rows, as if they're about to dance a reel and then the bride and groom walk down the aisle they have formed. This can be a great chance for some candid pictures. Be sure that none of the bridal party is blocking your view and that the flash does not bounce off an intruding shoulder or gown. Also, place any children at the front of the row so they will not be lost in the crowd.
The bride and groom's entrance procession is more formal sometimes and the bride and groom will walk between crossed swords, raised arms with hands linked together, or some other type of person formed enclosure. Get down low for this shot and include the special formation with the couple going through it.
Once the bride and groom have come through the group, have the party stand together for a quick group shot and back up enough so that you include some of the hall and especially the standing guests, in the picture. This should be a quick pose and it serves to show the presentation of the bridal party in the context of being received by the guests.
These events happen very fast, so be sure to have a fresh set of batteries, or a flash set on fast recycling time, for the whole series. You can not interrupt the flow of events by asking the group to wait or the maitre'd to slow his pace of announcing them just because your flash decides to lose power on you. This is true for many events during the reception, so be ready for the action.
The 'first dance' picture is one that exemplifies the need for discretion in wedding coverage. These are very intimate and touching moments. You can not rush in and pose people just because you need the picture. Be aware that soon after the bride and groom begin dancing the rest of the guests are invited to join in, so do not dawdle too long waiting for the right shot.
You can get a few good candid shots of the rest of the party at this time, but it might be better to get these shots as the average goes on and the crowd gets looser. Avoid shooting general scenes of various couples dancing but a shot of the group on the dance floor can be effective.