Here are some of the most common requested positions:
Collaborators: Collaborators are co-authors and co-developers involved in "your" case long-term. Although the details of each collaboration vary, accepting a collaborator means working on the case together and making it the case of all of you rather than just the original person. Collaborators will probably work with you on planning, design, and writing. It is advised if you request a collaborator that you have already produced a publicized and well-received case, and you have a concept for your case that you can share with potential collaborators. Be warned, as accepting a collaborator means giving another person control over your case!
Beta-Testers: Beta-testers will play through your case before you release it and give you their feedback. This allows you to make changes to improve your case before you release it publicly. It is highly advised that any case you write gets tested before you release it! The ideal beta-tester/reviewer already has experience in case development and is prepared to give you highly detailed information about their playthrough.
Reviewers: Reviewers are like beta-testers, but they make their comments after the case is released. Reviewers tend to be less detailed than beta-testers. If you're looking for something in particular from a review, be sure to say so!
Writers: Writers will create frames with minimal involvement in case-design. Most writers prefer to create their own dialogue, so it is important to clearly communicate to writers what kind of dialogue they should write.
You are not limited to this list and can request any kind of help, as long as it falls within this topic. You cannot bump a previous request until two weeks have passed since that previous request. After that, you must wait two months to bump it again. This has nothing to do with multiple requests at once, but it is appreciated if you do not overload the topic with these.
Happy trial developing, and good luck!