A study group works well for about 6, may 8 people. More than that and you will need to switch to a classroom format, or the group will just fall apart and not actually study the book.
Intermediate and advanced learners do not benefit as much from a study group, although they often are better helpers to the just beginning than the fluent speakers.
Tips for a good translation group
Pacing is important in translation groups because it can be very boring to spend an hour on one sentence. Pick a text that is easy enough to get through a few paragraphs an hour.
Looking up works in advance is good because dictionary lookups distrupt pacing.
If the text is interesting, then fluent attendees are less likely to be bored. Also, when someone fluent is at the table, take the opportunity to tackle more challenging texts.
Comics are a good choice because attendees are less likely to get bored should they hit a long section of text that can’t parse at all. At least with comics, you can follow part of the story regardless to textual understanding.
Poetry and many songs are not good choices for translation because they are very non-typical exemplars of text from a language.
Picking a language
Every langauge community is different. For example, you’ll get different people showing up at your events depending on how many immigrants there are, how the langauge community feels about bilingualism, if the langauge is famous, if the language is popular with academia (like Latin, Sanskrit or Basque)