Hi,

that's correct g_form won't work in g:evaluate as g:evaluate is server side

You can use current object

<?xml version="1.0" encoding="utf-8" ?>
<j:jelly trim="false" xmlns:j="jelly:core"
         xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">
    <a class="btn-default;" id="${jvar_n}" onclick="invokeChat('${ref}');">
        <img src="teams.png" width="25"  title="Teams Chat" alt="${gs.getMessage('Click to open Teams chat')}" />
    </a>
    <g:evaluate var="jvar_gr" object="true">
        var gr = new GlideRecord('sys_user_grmember');
        var assign_grp = current.getValue('assignment_group'); // use current object here
        gr.addQuery('group',assign_grp);
        gr.query();
        while(gr.next()){
        user += gr.user.email + ','; 
        }
        user; 
    </g:evaluate>
    <script>
        function invokeChat(reference) {

        var prefix = 'https://teams.microsoft.com/l/chat/0/0?users=';
        var firstname = g_form.getReference('u_requestor').first_name;
        var user1 = jelly.jvar_gr;
        var subject = '&amp;amp;topicName=' + g_form.getValue('number') +': ' + g_form.getValue('short_description') + '&amp;amp;message=Hi All, Can you please give me an update ?';
        var w = getTopWindow();
        var url = prefix + user1 + subject;
        w.open(url);
        }
    </script></j:jelly>

Regards
Ankur