For guests: read the envelope, and don't ask
Your invitation tells you whether you have a plus-one, and the signal is who's named: 'Jordan and Sam' or 'Jordan and guest' means yes, while 'Jordan' alone means the invitation is to you. When it's clear, don't ask for an exception — a host has usually set the guest list against a fixed venue count and budget, and pushing to add someone forces an awkward yes or an awkward no. Assume no plus-one unless the invitation says otherwise, and let the envelope be the final word.
For hosts: decide by a clear, consistent rule
The way to keep plus-ones from becoming a minefield is to set one rule and apply it to everyone in the same tier. A common, defensible line is to offer a plus-one to anyone married, engaged, living with a partner, or in a long-term relationship, and to invite single guests on their own — especially when they'll know plenty of other people there. Whatever you choose, be consistent: offering a guest to some people and not others in the same group is what actually causes hurt feelings, not the policy itself.
How to handle the awkward ask
If a guest asks for a plus-one you hadn't planned to give, you can decline warmly and without apology: 'We're keeping it small and sticking to named guests, so it'll just be you this time — I hope that's okay.' Naming the constraint (space, budget, a fixed count) makes it about the event, not the person. And if a guest is genuinely unsure whether the invitation includes someone, a polite one-line question to the host is far better than assuming and showing up with an extra.