You can definitely give guests a plus-one! When you add a guest to your Guest List, click "Add Plus One" to include a second guest. You can then check the "Name Unknown" box if you are not sure who they will be - don't worry, your guest can add their plus-one's name later when they RSVP. Lastly, click "Save & Close" or "Save & Add Another" to add this guest to your list.
Working from the App?
To add a plus one in the app just tap into the Guests + RSVP icon on the home screen > tap into the guest you want to edit > tap into the Guest Name(s) field > tap the "Add Guest" option. You can also choose the "Name Unknown" option in the app if you want guests to be able to fill in the guest name when they RSVP.
Note: We use a closed guest list tool on Zola, which means that guests must be on the guest list in order to RSVP. Guests are not able to add their own plus ones or additional guests in the RSVP flow, if you have not set up a plus one for them in your guest list ahead of time.