Out of curiosity?

alhlou

I haven't been to many weddings before and out of curiosity when sending out invitations are adding plus 1's mandatory? If not, do you just not mention it?

tiny speck

Not mandatory for people who are single, and yes, just address it to that person in that case, no need to mention you aren't inviting them with a guest. For people in relationships, you must invite their significant others, even if you haven't met them before. It's common courtesy.

OliveOilsMom

And any SOs should be addressed by name on the envelope.  It is also perfectly fine to send it to the person you know better, if they do not live together.  I received many invitations addressed to me and my H prior to us being engaged or living together.
Remeber that using the word "and" in between two people indicates marriage.  So for addresses with SOs, it would go like this:

Ms. Sally Smith
Mr. John Doe
Address
City, State Zip

itzMS

Remember to not "assume" someone is single. It can lead to hurt feelings. When in doubt, call and ask, as some people are not very public about relationships.

I personally think it is nice to always offer a plus-one to every guest, but I know not everyone can afford that.

bchinchilla

I'm not offering plus ones to people without relationships because of capacity at the venue.  What I did: 
-I didn't imply they could bring a date in the reply card.
-I spread by word of mouth that the reception is going to be a great single-mingle
-I sat my single guests near people they either know, or people who I feel they would be able to have fun conversations with. 
-If I know the person is shy and introverted, I gave them a plus one so they could bring a pal
-I avoided sitting singles at tables with all couples.  This can be a real downer for people who aren't enjoying their singleness.