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 will accept or deny it.
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If you're a team owner, go to the team name in the teams list and click More options
> Add member.
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Start typing a name, distribution list, security group, or Office 365 group to add to your team. You can also add people outside your organization as guests by typing their email addresses.
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When you're done adding members, select Add. Fyi, you can make someone a team owner by selecting the down arrow next to the word Member. There can be multiple owners in a team. For a deeper look at roles and permissions, see Team owner and member capabilities in Teams.
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Select Close. People you add to a team will receive an email letting them know they are now a member of your team and the team will show up in their teams list.