Parsing grammar of 我想不出还会有谁
In English, we might say something quite similar:


  I can't think of who else [we] will have.


The context might be someone organizing a party.  (Note, the "we" here is grammatically required in English, but not part of the Chinese.)

So maybe it's easier to understand if it were broken down in the following way:


  我想不出还会有谁。
  I [我] can’t think [想不出] of who [谁] else [还] [we] will [会] have [有].
  I can't think of who else [we] will have.


And (depending on context) it might be more natural to translate it to something simple:


  I can’t think of anyone else.


There are other ways to translate "I can’t think of anyone else" into Chinese (e.g. 我想不出其他人), and which is appropriate depends on context.