Shaykh Ubayd al-Jaabiree (hafidhahulllaah) was asked (source):

May Allaah be benevolent to you, this questioner asks: What is the ruling on giving lectures in the mosques of Ahl al-Bid'ah

The Shaykh responded:

Firstly: That which I know is that the people of innovations are of two types:

The first type: Those who are just, they accept the truth, even if their acceptance (of it) is not in a complete sense, but they listen to it and they do not show objection, and they leave the audience upon what they are upon. And these (likes) are those responsible for (running) the general mosques, to which they and others come to. So with respect to these mosques, we respond to the invitation so that the attendees from Ahl al-Sunnah can benefit, as this comprises support for them and strengthening them, and it also strengthens their resolve, and making them firm upon the Sunnah.

The second type: They divided into two (further) types:

A type whose mosques are closed (to others), no one but their own (type) come to them, and (overwhelmingly) they do not offer anyone who is not upon their way, the opportunity (to lecture), except when a (well-known) person of the Sunnah comes[1]. So delivering lectures in their mosques and their specific meeting places, this amounts to strengthening them and weakening the resolve of Ahl al-Sunnah. So responding to (their invitation) is futile, a mistake and is harmful upon Ahl al-Sunnah.

The other type: Those who speak and put themselves forward after the lecture has been delivered - whether the lecture remains present or after he (has left) and they give their own comments in which they throw doubts. So these (types of) mosques are not to be attended.

[1] The intent of the Shaykh here appears to be that some mosques do not give those who are Ahl al-Sunnah in their land the opportunity to speak and lecture in their mosques and they only allow their own types from the Hizbiyyeen. But when a prominent person who may be known for the Sunnah comes along, they may invite him, but this is to promote their own standing. So cooperating with these types of mosques is harmful to Ahl al-Sunnah in that it strengthens these Hizbiyyeen, because what is known from them generally speaking is closing of doors and allowing only their own types from the Hizbiyyeen to lecture. So cooperating with them, where that possibility might arise, harms the da'wah in reality as this only supports those people and their da'wah and gives them undue credibility. This in turn leads the people to be deceived thinking it is a place of the Sunnah.