You can use groomsmen or no one at all. We had no ushers and everyone knew how to find a seat on their own. To be honest, I think some of these "traditions" are just bullsh!t jobs these days.
Just let a few of the groom's men do it. I've only seen ushers, specifically, if there are more guys than girls in the wedding party. It's nice to have an escort to your seat. Especially for the aunts and other important family members that don't get formally seated.
We are having ushers, but pretty much because we have male family members we wanted to be a part of the ceremony (my brothers) but weren't really "groomsmen" material. So for us, it was perfect, but its certainly not necessary.
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I keep trying to tell FI and parents that ushers aren't necessary. Too much resistance from them. I have 2 brothers who are now 2 ushers. But we have a small church that will be fairly full, so maybe it's a good idea. I've never liked being seated by an usher personally.
It's a good way to include important guys in your life who won't be part of the wedding party (the bride's brothers/male cousins/close male friends perhaps?) But if you don't have them that's fine too. The groomsmen could easily fulfill the role.
Re: do you need ushers....