Have you ever had a client, customer or account that was important to you ask you to join their group?

If you make it a matter of policy not to try to combine personal and professional relationships, you can state that and not have it be particular to him. The thing is, if you already had such a policy, you’d have said so right away; now it’s a little awkward.

In your place, I believe I would say that his kind invitation has caused you to reflect on appropriate limits for your business relationships, and that as honored as you may be by his offering to extend this affiliation to you, you feel it would be wisest to keep those roles separate; or whatever words along those lines would suit you, as long as there’s no implication that his invitation was in any way out of line.

I don’t know what your business is, but I suppose it’s possible for you to meet clients or customers in another setting. How do you handle it then?

More simply put: “I’ve found it to be a good idea not to socialize with my clients. I was tempted by your offer, but in the end I have to regretfully decline.”

I do not have that kind of relationship with our clients.
I did get some ginger bread and a small bottle of mulled wine from one of them this christmas, though.

No. That would be awkward and unprofessional of them, I think. Just tell thank him for the offer, but politely decline. Don’t offer a reason. Reasons invite debate.

I have been asked, but as a rule I do graciously decline. I typically say that I am do not have time or space to take on any additional things like this. I also say that I would do a disservice to those I am already invested in by taking on new ones.

It really depends on the business and relationships. When we had an insurance agency, our partner was in organizations like the rotary and golf clubs for service and for relationship building. It’s not necessarily unprofessional to take part. But I agree with @Entropy that you don’t have to make a song and dance about it, just to say, “I appreciate the invitation and I’m sorry I can’t join your group.”

Yes.

But we’d just lost their case, and they were headed to prison. I thought joining him would complicate our professional relationship.

I think he understood.

Besides that, I agree with Jeruba. She’s handed you some pretty good lines. Use them wisely ; )

Personally I intermingle the two, business and personal. The social aspect is important in some careers more than others, like real estate where business is referred. But if you truly have no interest after researching, I’d just be honest if asked again.