1) Export ParentId* to whom you want to follow as
a) Launch the Data Loader
b) Use "Export".
c) Login using Salesforce credentials.
d) Check off the "Show all Salesforce object".
e) Choose User.
f) Choose the Target for extraction and make sure to specify the name of the export file and its location.
g) Click Next
h) Select at UserId.
2) SubscriberId* is the Id who wants to follow all users.Like you wants to follow all users hence, you have to put your Salesforce id.
3) Create one CSV using these two fields SubscriberId and ParentID( UserId) and save this.
4) Go back to Data loader and click Upsert.
a) Check off the "Show all Salesforce object".
b) Choose Entity Subscription (EntitySubscription).
c) Browse your file, Click next.
d) Map the ID User Id with ParentId and SuscriberId with SubscriberId. Click on Ok.
e) Select the directory where your success and error files will be saved.
* ParentID refers to the record which the user is following.
* SubscriberId refers to the User who is following the record
Note: Maximum limit to follow records in Chatter is 500 records.