Add-on Custom Userfield and Group Promotion…

Some advice please if I may.

Our forum runs for the members of a vintage motorcycle club, which has about 2500 paid-up members worldwide.

All people who are members of the Club can join our xenForo forum, and when they do, they include as one of the fields their 7 digit club membership number.

They then e-mail in, and we check to see that their membership number is valid and paid up. If they are, and all is OK, we assign them a special usergroup which has access to all areas of our forum.

Also, some non club members who have an interest in our motorcycles are welcome to join the forum, but as they don't have a membership number, they only get normal access to the 'Registered User ' usergroup and as such, they don't get access to some of the enhanced features.

OK, so far, so good. Now the tricky question.

Can you think of any way of automating the 'upgrade' process?

At the moment, I have a spreadsheet of all the paid up members with their membership numbers on, but clearly it can be a time-consuming process.

xenFofo support said that it would be possible to automate the process by doing up a lookup of the membership ID which could be entered in a custom user field, and then applying a user group promotion if it is valid.

Unfortunately, this is way out of my league and ability (I'm an old man who rides an old British motorcycle), and wondered how much I would expect to pay for something like this, and who should I approach?

Look forward to hearing from anyone!