It’s best to use a speedlight, not your camera’s pop-up flash, but the speedlight alone will probably give you faces that are too bright, backgrounds that are too dark, and light that is too harsh. At weddings, parties and fundraisers, friends and families are together for a short time and they are surprisingly willing to be in a photograph.
Using an on-camera flash at an indoor event is probably the simplest way to make pictures of small groups look their best.