Not an expert, but …
There seemed to be no built in funcionality for this in Identity and I could not get it work from built in Roles also (it seems to not work with claims based Identity).
So I ended up doing something like this:
var users = context.Users
.Where(x => x.Roles.Select(y => y.Id).Contains(roleId))
.ToList();
x.Roles.Select(y => y.Id)
gets a list of all role ids foruser x
.Contains(roleId)
checks if this list of ids contains necessaryroleId