am unable to get selected item from selectlist and passing to controler  bellow is my code please help me on this

vf page
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<apex:page standardController="user"   extensions="Jobformation1usercon">
       <apex:form >
       <apex:pageBlock >
         <apex:pageBlockSection >
           <apex:pageBlockTable value="{!accwraplist }" var="accwrapref">
       
      
       <apex:column headerValue="Job Name" value="{!accwrapref.acc.name}" />      
        <apex:column headerValue="User" >
           <apex:selectList size="1"  rendered="true"  value="{!a}">
             <apex:selectOptions value="{!Value}" >
                               
               </apex:selectOptions>
                 </apex:selectList>
         </apex:column>
          <apex:column headerValue="Share a Record" >
                          <apex:commandLink action="{!manualShareRead}" value="Share">
                     <apex:param name="id" value="{!accwrapref.acc.id}"/>
          </apex:commandLink>
                   </apex:column>
            </apex:pageBlockTable>
         </apex:pageBlockSection>
       </apex:pageBlock>
      </apex:form>
</apex:page>
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apex controler
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public class Jobformation1usercon
{

         List<user> PGList= new List<user>(); 
         public List<job__c> jl {get;set;}
         public user gp {get;set;}
         public boolean nojb{get;set;}
         public String a  {get;set;}
         public List<job__C> acclist {get;set;}
         public List<Accountwraper> accwraplist {get;set;}
         public List<group> lstgroup {get;set;}
  


        public Jobformation1usercon(ApexPages.StandardController controller)
       
       {
    
        accwraplist =new List<Accountwraper>();
        acclist=[select id,name from job__c];
        Accountwraper  objwrap;
        for(job__c objacc:acclist)
        {
        objwrap=new Accountwraper();
        objwrap.acc=objacc;
        system.debug('acclist value'+acclist);
        accwraplist.add(objwrap);
        system.debug(' objwrap value '+objwrap);
        }
       
        //
    }
     public List<SelectOption> getValue()
     {  
         List<SelectOption> option = new List<SelectOption>();
            
        for(user gp: [Select Name From  user  ])
         {
         
             option.add(new SelectOption(gp.id,gp.Name));
             system.debug('option value is '+option);
            
         }
         return option;     
    }
                
       public PageReference manualShareRead()
           {
             
               Id id = System.currentPageReference().getParameters().get('id');
               system.debug('Id Value is'+id);
               system.debug('aaaaaaaaaaaaaaaaaaaaaa'+a);
               PGList = [select id from user where name=:a];
               system.debug('PGList value is'+PGList);
               Job__Share jobShr  = new Job__Share(); 
               jobShr.ParentId = id;
               jobShr.UserOrGroupId = PGList[0].id;
               jobShr.AccessLevel = 'Read';
               jobShr.RowCause = Schema.Job__Share.RowCause.Manual;
              
             Database.SaveResult sr = Database.insert(jobShr,false);
              system.debug('sr value is'+sr);
      
             if(sr.isSuccess())
            {
               system.debug('sr.isSuccess() value:'+sr.isSuccess());
                  system.debug('if sr.isSuccess() block');
               PageReference nextpage = new PageReference('/apex/CustomTeam?id='+id);
             
                return nextpage;
               }
           else {
                  system.debug('inner else sr.isSuccess() block');
                     Database.Error err = sr.getErrors()[0];
                     if(err.getStatusCode() == StatusCode.FIELD_FILTER_VALIDATION_EXCEPTION  && 
                     err.getMessage().contains('AccessLevel')){
                      system.debug(' iner if sr.isSuccess() block');
                      PageReference nextpage = new PageReference('/apex/CustomTeam?id='+id);
                return nextpage;
            }
             else
             {
                      system.debug('outer else sr.isSuccess() block');
               PageReference nextpage = new PageReference('/apex/CustomTeam?id='+id);
                    return nextpage;
               }
             }
         
         }
  
  
        class Accountwraper
        {
        public job__c acc { get;set;}
        public boolean selected { get;set;}
       
        }
     }