Regroup is not normally used to mean resume a meeting. It generally has the connotation of reorganizing after something bad has happened. "The battalion regrouped after a quarter of the men had been killed in the attack."
I would use something like "We will meet back here after lunch," or "...resume after lunch". If I absolutely had to have a single intransitive verb that meant come back together (for another session of a meeting), it might be "reassemble", but I don't like it very much.