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If they are in your wedding party, I would.
But otherwise, if they are truly unattached singles, it's a nice gesture but not required.
We have a group of 7 friends that were all single at the time our invites went out. We didn't give them +1. Mainly because we didn't have the budget. I didn't feel bad about not giving them a +1 because it's not like they wouldn't know anyone there. They filled up a whole table with just the 7 of them. I did have a truly single friend who wasn't part of that group that was invited and she didn't know anyone there besides me. She is kind of shy and obviously I couldn't hang out with her the whole night. So I gave her a +1 so that she could bring someone with her to talk to and hang out with and make her evening more enjoyable.