How to add a member to teams by a Non-Owner user ?

Accept or deny requests to join a team

People can submit a request to join your team.

https://support.microsoft.com/en-us/office/accept-or-deny-requests-to-join-a-team-94d538c6-e3c8-4471-9780-05c9c5282024

Add members to a team in Teams

If you're a team owner, you can add someone to a team. If you're not a team owner, you can submit a request and the team owner can then accept or deny it.

https://support.microsoft.com/en-us/office/add-members-to-a-team-in-teams-aff2249d-b456-4bc3-81e7-52327b6b38e9

Team owner, member, and guest capabilities in Teams

Every member in Teams has a role, and each one has different permissions.

https://support.microsoft.com/en-us/office/team-owner-member-and-guest-capabilities-in-teams-d03fdf5b-1a6e-48e4-8e07-b13e1350ec7b