Mazel tov and congratulations on your upcoming marriage! Choosing witnesses to sign your ketubah is a deeply personal decision.
In Reform Judaism, witnesses may be of any gender, above b’nei mitzvah age (13 or older), and customarily, identify as Jewish, although some clergy permit individuals from other backgrounds and faiths to serve as ketubah witnesses. Some clergy also will allow additional witnesses, so you can honor three or even four friends as witnesses.
Many, but not all, clergy adhere to the rule that witnesses not be related to either wedding partner or to one another. In other words, choosing witnesses is a unique opportunity to honor and include beloved friends. Be sure to consult with your officiating rabbi or cantor regarding their practices regarding who may serve in this role.
Once you know the clergy’s parameters, consider the people you might like to honor in this way and who would most appreciate this role. Perhaps it is someone who has an especially strong and loving marriage, a best friend who knows you to your core, a friend who’s like family, or someone who had a role in bringing the two of you together.