Quick answer
Set one rule and apply it consistently — most hosts use "partners by name, plus-ones for guests attending alone". State the allowance on the invitation itself, and always collect the plus-one's name so they exist in your headcount, catering and seating.
Pick a rule you can defend
The arguments about plus-ones are never really about seats, they are about consistency. Any of these is fine as long as it applies to everyone:
- Named partners only — anyone in a relationship you know about is invited by name
- Partners plus solo guests — anyone attending alone may bring someone
- Everyone gets one — simplest, most expensive
- Nobody does — acceptable for small events if you say so plainly and early
What causes offence is not the rule, it is discovering that someone else got a different one.
Say it on the invitation, not in a follow-up
Ambiguity is what generates the awkward message asking whether they can bring someone. Make the allowance explicit and the question disappears.
Always collect the name
An unnamed "+1" is a person your caterer cannot plan for and your seating plan cannot place. Ask for the name in the RSVP itself, at the moment the guest is thinking about it — chasing it later is a second conversation you do not need.
In Inviterio a plus-one becomes a real guest record with their own meal choice and dietary notes, so they show up in the headcount and on the seating chart like everybody else rather than as an asterisk.
Per-guest allowances, not a global switch
Plus-one permission is best set per guest. That is what lets you invite your cousin and her partner by name, give your single friend a plus-one, and invite your colleague alone — without three different invitation templates.
The awkward cases, handled
Someone asks for a plus-one they were not offered: answer honestly and early — "we are at capacity with the venue" is both true and final. Someone brings an uninvited guest anyway: that is a day-of problem for whoever runs the door, which is a reason to have a check-in list rather than a reason to relitigate the rule.
A plus-one who cancels late is simply a decline against their own record — which keeps your numbers right without you subtracting anything by hand.