What are you trying to do and what problem are you having?
Whichever user created a group can send invitations to join the group, by email or Zotero username, from the group's member settings https://www.zotero.org/support/groups#members_settings If the group is public you can also just send a link to the group's zotero.org page and they can join (or request access) from there.
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Whichever user created a group can send invitations to join the group, by email or Zotero username, from the group's member settings https://www.zotero.org/support/groups#members_settings
If the group is public you can also just send a link to the group's zotero.org page and they can join (or request access) from there.