...
from django.contrib.auth.models import User

class projects(models.Model):
    name_proj    = models.CharField(max_length=255)
    mkey_user    = models.ManyToManyField(User)
....

....
form = AddProjectForm(request.POST)
if form.is_valid():
    project = projects(name_proj  = form.cleaned_data['name_proj']).save()
...

 project.mkey_user.add(form.cleaned_data['mkey_user']).save()

projects.mkey_user.add(form.cleaned_data['mkey_user'])

Exception Value:
Manager must be accessed via instance

projects.mkey_user.add(form.cleaned_data['mkey_user'])

a1 = User.objects.get(id=request.POST['mkey_user'])
projects.mkey_user.add(a1)