The company has no business being interested in what you do in your spare time. As long as you keep this outside of work hours, there isn't really much they can do to stop you. However in the interests of not appearing to be in open rebellion against the company you should probably keep this relatively discreet, by which I mean don't put up posters advertising the meetup, and use company email with discretion. Don't send invitations to mailing lists where senior managers or HR might be copied. (However also don't assume that immediate managers are against this meetup - some of them might be interested in joining you).
I would recommend waiting a week or so before meeting. Those let go are going to be understandably angry in the first week or so, no matter how well the layoffs were handled, and during that time any conversation is going to be mostly about what utter, unforgivable, unrelenting bastards the company are. Sometimes that isn't healthy, for either the survivors or non-survivors.
It will also be necessary to be the tiniest bit discreet about company information. Ordinary things you would have shared before shouldn't be a problem, but if the company cancelled a major project, or gave employees financial information right after the layoffs, don't forget they are no longer part of the company.