Currently there is no way how to force new community members to follow a community after they have been added to it. Only new community owners follow communities automatically.
This is a serious issue in scenarios that require new members to start following a community (project communities, formal top-down comunities etc.) Especially novice or external users often forget to click on the “follow this community” button and therefore become excluded from all updates unless they are @mentioned
Please add new settings “force new members to start following this community” to enable/disable auto-follow per community. This should be one-time action which switches on the follow feature when (and only when) a user is added to a community. The user shall be always able to intentionally stop following the community.
If you found such feature as too “dangerous” in hands of all community owners (risk of misuse), then you can limit it via a security role communities-admin or something similar.
This should work the same regardless the affected users are internal or external.
Second, please consider to create also a API call for this.
Third, it would be nice to provide community owners with information regarding which members follow the community. Perhaps it could be easily implemented via a new drop-down filter on the Community -> Members page.