Sample Code:global class SubscribeOrUnsubscribeLead {
    webservice static String subscribeLead(Id leadId) {
        List<EntitySubscription> listEntitySub = new List<EntitySubscription>();
        listEntitySub = [SELECT Id FROM EntitySubscription WHERE ParentId =: leadId AND SubscriberId =: 
                                        UserInfo.getUserId()];
        if(listEntitySub.size() == 0) {
            EntitySubscription entitySub = new EntitySubscription(ParentId = leadId, SubscriberId = UserInfo.getUserId());
            insert entitySub;
            return ‘Subscribed’;
        } else {
            return ‘You have already subscribed’;
        }
    }
    webservice static String unSubscribeLead(Id leadId) {
        List<EntitySubscription> listEntitySub = new List<EntitySubscription>();
        listEntitySub = [SELECT Id FROM EntitySubscription WHERE ParentId =: leadId AND SubscriberId =: 
                                        UserInfo.getUserId()];
        if(listEntitySub.size() > 0) {
            delete listEntitySub;
            return ‘Unsubscribed’;
        } else {
            return ‘You are not subscribed to this’;
        }
    }
}

Subscribe button code:

{!REQUIRESCRIPT(“/soap/ajax/15.0/connection.js”)}; 
{!REQUIRESCRIPT(“/soap/ajax/15.0/apex.js”)}; 
alert(sforce.apex.execute(“InfallibleTech.SubscribeOrUnsubscribeLead”,”subscribeLead”,{leadId:”{!Lead.Id}”})); 
location.reload(true);

Unsubscribe button code:

{!REQUIRESCRIPT(“/soap/ajax/15.0/connection.js”)} 
{!REQUIRESCRIPT(“/soap/ajax/15.0/apex.js”)}
alert(sforce.apex.execute(“InfallibleTech.SubscribeOrUnsubscribeLead”,”unSubscribeLead”,{leadId:”{!Lead.Id}”})); 
location.reload(true);

Output: