If You’re a Guest:
Unless you are given permission by the bride and groom personally to bring a guest, if the invitations do not state that you can bring a guest, then you simply cannot bring someone as a plus one. If you think you are misunderstanding the wording on the invitation, then you can ask the couple for clarification. Chances are it doesn’t say ‘and guest’ or ‘plus one’ on the invite, then you do not need to find a date to escort you.
While it would be nice for you to bring your significant other to your friend’s wedding if the invitation doesn’t say you can, that doesn’t mean to call up the bride and groom to ask for special permission to do so. Respect the couple’s decision to not allow everyone to bring a plus one, asking them if you will only put them in an awkward position and it is their day, not yours.
If the invitation does say you can bring a guest, wedding plus one etiquette suggests this primarily means your significant other. Avoid asking your roommate, friends or siblings to attend the wedding with you, and do not consider asking someone you know will either be disrespectful or may not be able to attend.
If you're single or just got into a new relationship, do not be offended if the couples didn't include a plus one for you. Weddings are a great expense and you should be understanding that you being able to bring someone so that you don’t feel alone or uncomfortable is absurd. You are there to celebrate the couple’s big day, so you will want to put your ego aside and not make big or even small deal about it.
Do not RSVP with a plus one if you are unsure if your date can make it. You should always RSVP with the name of the guest you are bringing, so they are certain that you are bringing a guest. This also gives the couple a bit of relief in knowing they are not just paying for a stranger to attend their big day.
If you receive an invitation and it is addressed to you and a specific guests name, most likely your significant other, and they can’t make it, this is not a free pass to invite someone else. The invitation was addressed to you and that person who is the couples want in attendance. If the other person cannot make it, then you will have to go alone.
You should not ask your date to pitch in for the wedding gift, especially if they barely know the couple. You can add their name to the gift if you choose, but it is tacky and improper wedding plus one etiquette to invite someone to a wedding and then ask them to help out with the gift.
The couple is paying for your plus one to be there, so it is polite that you introduce them to one another at some point during the reception. But, remember this is a special day for the bride and groom, if they aren’t paying enough attention to you and your date, don’t take it personally or make the night about you.