The other two candidates standing in the Napier electorate - Alliance candidate Mary O'Neill and Barry Pulford of the Democrats for Social Credit - also attended, along with representatives from other parties not standing candidates in the seat.
"We wanted everyone to have a good hearing. If we had them all at once it would have been hopeless," Ms Boag said.
"It would be unfair to invite Mr McVicar to speak at today's meeting because his party had already been given a voice at an event organised by the organisation, which has about 600 members.
"If we'd got him in [today] he would have been double-dipping. We've already heard from the Conservative Party and they're not in Parliament at this stage - that was the criteria for these three [speaking today].
"We didn't know when we organised this, a few months ago, where Garth McVicar's polling would go."
Ms Boag said Grey Power had no affinity with any particular party and the Napier and Districts committee organising today's meeting included members of a number of parties, including one member of the Conservative Party.
"We're sorry we can't have him [Mr McVicar] tomorrow but it's just the way we organised it - we're trying to be fair to all the candidates."
Mr McVicar said the organisation's decision was disappointing but he would respect it.
"Ultimately they underestimated the impact my entering the race was going to have, which disappointed me a lot because Grey Power have been huge supporters in my previous life [fronting the Sensible Sentencing Trust] and we've achieved some fantastic changes together," he said.
"I understand where they're coming from - they made their decision some time ago. I respect their decision."
Last month Conservative Party leader Colin Craig went to the High Court to force TV to include him in a debate between minor party leaders.
He was initially excluded because the broadcaster wanted to limit participation to leaders of parties that had been in Parliament over the past three years.
Mr McVicar said he would not be taking any action to force Grey Power to include him in today's meeting.
"That's not my style. I won't be doing that at all."