I dont think there is any direct way to do it. I asked about that to google support and their reply is :
If you want that anyone should be able to join the group then you have to change the group settings to Who can join the group and select the option Anyone on the web can join i.e. Anyone on the web can add themselves to the group. If you’re signed in to a work or school account, this option appears only if your administrator has set the option to let group owners allow external members.
The users can join the group with below given steps
Join a new group
Sign in to Google Groups.
Click All groups and find the group that you want to join.
Click Join group.Note: If you don't see the "Ask to join group" option, you can email the group and ask to join it.
In the Subscription section, choose how often you want email updates from the group:
Every new message
Send daily summaries
Combined updates
Don't send email updates
Click Join group
Also if you want to send the invitation to join the group where the user will receive the link to join the group then the setting Who can join group Choose an option: Only invited users—People have to be invited to join the group. They can't add themselves directly or ask to join.
Kindly refer the articles https://support.google.com/groups/answer/2464926?hl=en#zippy=%2Csettings-reference%2Cadvanced-settings-reference
https://support.google.com/a/users/answer/9304806?hl=en#1.2