A good rule of thumb for putting plus one on an invite is the old standby, “no ring, no bring”. This simply means if the person you are inviting isn’t married or engaged then there is no need to include a plus one on their invite, especially if the budget and space is an issue. Generally you can also include a plus one if the person is living with their significant other and just hasn’t gotten engaged or if someone is in a same sex relationship and marriage is not a possibility. If you have a little room on your guest list you can include any significant others that are long term or the couple knows by name. You can always call your guest and ask if there is someone special you should be inviting for them. The important thing is to set boundaries and stick by them when people RSVP and ask about bringing a date. Just keep the singles seated together at the reception where appropriate. One last thing to consider is whether a single will know anyone else at the event. If they are related or will know many of the other guests there is really no need for an exception to your boundaries. It is good form but not mandatory to allow all of your attendants a plus one.