Prevent adding user to multiple ordergroups

[twothreenine]

Since it is not intended for a user to be in multiple ordergroups and this leads to various errors like #631, an error message should appear when a user tries to add a user who is already in an ordergroup to another ordergroup (either by mistake or by not knowing that a user must not be in multiple ordergroups).

[aban1]

I will be attempting this soon :)

[kidhab]

Can you explain how to add someone to two ordergroups? Playing with the Demo I can't reproduce it.

[twothreenine]

Can you explain how to add someone to two ordergroups? Playing with the Demo I can't reproduce it.

There is no error message when you do that while creating an ordergroup.

[aban1]

Can you explain how to add someone to two ordergroups? Playing with the Demo I can't reproduce it.

I don't know the password to the demo website but running it locally when I create two order groups with the same admin account, the account joins both. Here are the screenshots below.