I'll play Devil's Advocate here and take the negative side for a minute...
Adding a new player to a group can be a tricky, and can end poorly if done improperly. In the best case scenario, the new player has to create a new character, meet new people (in and out of the game), work themselves into a role in the party, and learn the groups houserules. In the worst case scenario, the new player has to learn how to play D+D for the first time as well. And the new player also has to have fun doing this if you expect them to play again. No one should be thrown into the middle of a group/campaign just to "see what D+D is like".
If this is the first time the significant other, I recommend having them be around for one gaming session before they actually create a character and join in with the group. This is especially true for the "see what it's like" scenario, as watching takes a lot less effort than playing. If the session is being hosted at the place where they live, it makes this much easier and less awkward for everyone. If things seem to be going well, the new player could start by taking control of a NPC or two and assist the DM. They can then start a new character for the next session, after having seen how the rest of the party works.
Under no circumstances should a significant other be brought into the group unless they really want to be there. And they should never be half-playing and half-hosting (or half-babysitting, or half-anything else).