Here are the rewritten sentences:

1. I joined the volunteer club because I wanted to help people in need. (so)
→ I wanted to help people in need, so I joined the volunteer club.

2. Volunteers collected marine litter on the beaches. (picked)
→ Marine litter was picked up by volunteers on the beaches.

3. Let me reach my office and then I will talk to him. (when)
→ I will talk to him when I reach my office.

4. Why don't we donate our old books, toys, and clothes to charity? (suggests)
→ Mom suggests that we donate our old books, toys, and clothes to charity.

5. Jane stayed behind after the class in order to talk to the teacher. (because)
→ Jane stayed behind after the class because she wanted to talk to the teacher.

6. My sister participated in volunteer work two years ago. (part)
→ My sister was part of volunteer work two years ago.

7. How long have you been a member of Volunteers In Asia? (join)
→ When did you join Volunteers In Asia?

8. We suggested people recycle glass, cans, and paper. (encouraged)
→ We encouraged people to recycle glass, cans, and paper.