Silversea has allowed guests in the past---provided you get prior approval. We have had sons in Europe join us for dinner when the ship was docked near London Bridge.I don't know if they still allow this.
In my experience Seaborn (140 days cruising on 12 or more cruises) does not allow guests---unless they recently changed their policy. Since 9/11 cruise lines are very fussy about who they allow on board---and rightly so.