Good to know it is working. Be sure to mark it as correct so that others with a similar issue can find the answer quickly.

Let me walk through each line of code in the function of the script include:

               var grpID = this.getParameter('sysparm_grp'); //This is the parameter passed from the Client Script

               var usrID = this.getParameter('sysparm_usr'); //This is the parameter passed from the Client Script

               var memObj = new GlideRecord('sys_user_grmember'); //initializes a GlideRecord object on the Group Membership [sys_user_grmember] table

               var qString = 'group=' + grpID + '^user=' + usrID; //Builds a query string to find the group and user in the Group Membership table.

               memObj.addEncodedQuery(qString); //Adds the query string using the addEncodedQuery() method

               memObj.query(); //Runs the query

               return memObj.hasNext(); //uses the hasNext() method to indicate if the query has any found records.

Some helpful documents to refer to:

GlideRecord - Global

GlideRecord - Scoped

AJAX