Unfortunately it's not really possible to understand what the issue is or provide a good solution unless you provide more details.
You may not be the owner of the teams that you want to add members to. Guest access might be disabled in your organization. There might be "collaboration restrictions" for your organization and the domain(s) of the guests that you try to add might not be allowed there. There might be lots of other causes.
I'd recommend you to contact your organization's IT department or Office 365 admins for further assistance. If you are already a member of those, please provide more details.
https://support.office.com/en-us/article/add-members-to-a-team-in-teams-aff2249d-b456-4bc3-81e7-5232...
https://support.office.com/en-us/article/add-guests-to-a-team-in-teams-fccb4fa6-f864-4508-bdde-256e7...
