Our suggestions for inviting partners is as follows:-
Are they married? If so then you should try to invite them if possible.
How long have they been together? If they have been together for 15 years but just never married then they probably deserve an invite, but if they only started dating in the last 6 months then they can be left off the list.
Have you meet the partner more than once? If you have meet a couple of times then an invite would be nice, if you have never meet or only met once than an invite is not necessary.