I've been the spouse in this situation.
My husband and I recently attended the wedding of one of his best friends growing up. I only had ever met the friend and his wife before (at our own wedding) and everyone else was unknown to me. This is maybe a bit more extreme than a regular meeting of friends but it's a starting place.
What I found in that situation was a couple of things:
If they don't know anyone, how well your SO is able to react/interact with these people will depend on your SO.
I'm not a joiner, really. I wanted to go to be with my husband but I left some of the festivities early when it seemed like things were devolving into walks down memory lane and boozing (I'm not a fan of drunk people). But I did enjoy some of the chatter early on when they were each telling their partners about the stuff they got into together. The more interested your SO is in getting to know these people and what interests you, the more able they will be to integrate to some degree.
Some people love being involved... so if your SO is more extroverted and willing to wade in and get to know people, they will likely have more fun than if they're introverted and tend to stick with you. If there are other SOs there, that will help. They can commiserate about feeling unsure what to do with themselves at the party. Or they can talk about things they actually enjoy.
If they don't know anyone, how much the other people want to include your SO will affect their experience.
At the wedding's rehearsal dinner I mostly got introduced to old friends and chatted about them, their families, what they were doing now... passive "catching up" sorts of things, as you might expect from people who haven't seen each other for the better part of a decade. But at the wedding, one of my husband's old friends really wanted to talk to me and include me and get to know me and that made me feel welcome and wanted and interesting. It was so nice for a change. But I don't know if this personality type is common. I was able to open up and share and laugh and not feel on the outside.
So, know your SO and know your cohort.
Your first step is to ask your SO this question... find out what they'd like... set a game plan. If your SO is quiet and reserved, they may prefer that you stick with them until they feel comfortable. If they're more likely to be one to be able to fend for themselves, see how they'd like to get introduced and where to go from there. But we can't tell you which of these your SO wants... only they can.
If you have a member of your cohort who you think might have something in common with your SO, match them up. I studied film and one of my spouse's friends did voiceover work and photography... we had a common interest to start from.
Stay in contact and be respectful if they want to leave.
The last thing you want is to force your SO to be somewhere they don't want to be. Check in with them regularly (hourly?) and consider options for leaving either separately or together. If the party is near your home, consider dropping them off at home if they get tired before you're ready to leave and going back yourself... if it's far away, maybe take two vehicles (if you have them) or consider seeing if there's an alternate way home for them. If this is a regular event, maybe you just decide that this first one will be a test and if your SO isn't clicking, you both go home.
If you sense that they want to head out, don't ignore them and keep chatting for another 30 minutes... find a solution.