The more students can work with materials before discussion to prepare, the better. Presenting a great deal of unseen material or a great number of new concepts in class can be hard for students to understand, absorb, or discuss in a meaningful way. One way to offer adequate time is to employ the "flipped" classroom technique, in which students review and work with materials before they are discussed in class. This approach can be scaffolded, too, with materials being used in one way outside of class then further explored or expanded on with an in-class activity.
There are several ways an instructor can make materials more accessible and discussion easier for NNS to take part in:
Study guides or questions
Providing students with guiding commentary or questions allows them the time to consider the text/reading and understand better what they need to know or prepare for discussion. If students have the time to craft answers to questions before class, they are much more likely to participate, particularly in pairs or smaller groups. An instructor can also direct students' attention to particularly important passages or concepts through comments and questions, which can also be helpful to NNS students who tend to read at a much slower pace and may not be able to differentiate between what is vital in a reading and what is not.
Encountering new vocabulary in nearly every sentence can both slow a student's reading process and complicate comprehension. Even with a dictionary in hand, a student may not be able to determine which definition of a word is appropriate in a given context. By providing glossaries for readings with complex or subject-specific vocabulary, an instructor can zero in on the definition that is needed for quicker comprehension.
Requiring students to complete tasks related to a reading before a discussion (either alone, in pairs, or in small groups) gives students a chance to work through their understanding of a reading/concept by applying it to the task. This can be done as homework, then brought to class to be discussed in pairs or small groups to ensure students have arrived at the desired conclusion or can apply their knowledge correctly.
Giving students some time before discussion to reflect on a reading or an idea and write down their thoughts is also good preparation for discussion. They can first "pair and share" what they have written and then join a larger discussion having already gained some confidence. Instructors can also have students "pair and report" which allows students to talk about what their partner said, which can feel less intimidating than sharing one's own opinion, but also ensures students are really listening to each other and negotiating meaning.
Instructors can have graded discussions in class, handing out a rubric beforehand to let students know what good discussion participation looks like. This can be helpful as students prepare for discussion because they can have appropriate comments or questions ready to share. It is also helpful to have a rubric that includes a description of what bad discussion participation looks like, particularly the idea that dominating a discussion and/or ignoring those who are silent isn't rewarded in grading. Calling on students to be good listeners and leaders who bring people into discussion can help NNS students participate since they have a much harder time jumping into a fast-paced discussion than domestic students.