Usually, yes.

Whether another guest can be added depends on:

• Cabin occupancy limits
• Cruise line availability
• Applicable pricing
• Government requirements

If your cabin is rated for double occupance, you will also have to change cabins one that allows for 3 or 4 people. 

Changing roomamates may be possible, but each cruise may have specific limitations, and changes may not be possilbe after the ship manifest is finalized, usually 1-2 weeks prior to the sailing. 

There is also typicaly a change fee.  Note that once one roommate has been changed, further changes can only be made to the "new" roommate; in other future changes only apply to the new person and not the original roommate such that at least one person on the original booking must remain. 

Contact us as soon as possible if you'd like to add someone to your reservation, as availability becomes more limited closer to sailing.