Since this question was posted and a best answer selected, there has been a development within the Jehovah's Witnesses religion. I only add this as information to supplement the answers already given, that correctly state their meetings to be open to the public.
At their 2016 Assemblies, also open to the public, Jehovah's Witnesses began to receive preparatory instructions and information about a time to come when they would need to gather together in small groups, out of sight of the public, to be directed by their local elders who would convey instructions from those they were responsible to. Local elders would receive directions from the Governing Body in America to be in regular contact with their congregation members on a weekly basis.
This new development is based upon their beliefs about 'the Great Tribulation' which they say will start any time now. They are being told how to prepare for it, by planning to bunker down in hideaways in private locations. They are to unquestioningly obey whatever their elders tell them to do. https://www.youtube.com/watch?v=wNtlDybBVA8 for all of the videos.
A very recent instruction received by Jehovah's Witnesses came through on 16 April 2019, based upon their forthcoming study of the July 'Watchtower' magazine. I copy the text here:
Something encouraging for all of us. Each day brings us closer to the
vindication of Jehovah God's name and Paradise. The Governing Body has
written two studies of the Watchtower, to prepare us for the
persecution. We will have them on September 8 and 15/19. The articles
contain practical advice, as:
1-Meetings in small groups.
2-Wear clothes that do not attract attention.
4-Do not leave publications when we preach.
5-Hide some publications at home.
6-Memorize from now on songs that strengthen us.
7-Know how to keep quiet.
8-From now: constant prayer.
9-Know Jehovah well to show our trust in him at all times.
The Watchtower of July 2019. Link for the first study
I received this as a Screenshot_2019-04-16-20-03-53.png
Because Jehovah's Witnesses believe that world governments will turn on world religions very soon now, they anticipate the need for them to hide away, out of public view and attention, for they will also be targets. They think that once the governments turn on them, Jehovah will start the battle of Armageddon. This preparation for the start of 'the Great Tribulation' which will lead into Armageddon, will require very private meeting together to which the public will not be invited. It is even possible that mention of this here, on a public forum, will result in pressure to have this answer removed. But I give it, as an update to answers that did not include this development - for completeness.