It depends a lot on what kind of forum you want to run.
Shared hosting may be enough for a very small forum, but 100 active users can mean very different things depending on the setup.
A few important questions:
-Will users be able to upload attachments/files/images?
-Will the forum host many images or downloads directly?
-How many guests/bots do you expect besides registered users?
-Do you want to use XenForo search only, or Enhanced Search/Elasticsearch later?
-Do you plan to use many add-ons?
-Will you use Cloudflare or another CDN/cache in front?
Is “100 active users” meant at the same time, or 100 members per day?
-What PHP memory limit, CPU/RAM limits and PHP modules does the shared host provide?
For a simple new forum without file uploads and without heavy add-ons, shared hosting can work at the beginning.
But if you expect 100 active users at the same time, uploads, many images, bots and future growth, I would recommend starting with a small VPS instead of shared hosting. You do not need an expensive dedicated server, but shared hosting may become limiting quickly.
