If you are a delegate of someone else and you have access to their calendar (they have shared their calendar with you), you should be able to schedule meetings on their calendar (in outlook) and either you or the calendar owner can start that meeting. You have to go to their calendar in outlook and select the meeting you set up and click join (It will not be available in your Teams app calendar if you didn't invite yourself, as Teams only shows your own calendar).

To share their calendar with you, please see: Share your calendar in Outlook - Microsoft Support (They should select delegate Permission when sharing).

Share your calendar (they should share their calendar with you).

1. Select Calendar > Share Calendar.
2. Choose a calendar to share.
3. Select Add, decide who to share your calendar with, and select Add.
4. Select OK and you'll see the added people with a delegate permission level.
5. Choose a name, select the access level to give, and select OK.

Open a shared calendar

1. Open an email with a shared calendar and select Accept.
2. In Outlook, select Calendar > Shared Calendars to view a shared calendar.
3. Choose a calendar to open.
4. Right click on the calendar's name and select Overlay to layer the shared calendar over your own.

Once you have access to their calendar, you can schedule meetings, join their meetings....

Another workaround would be, when you create a Teams meeting, add another person as co-organizer, and they can add you as co-organizer when they schedule the meeting.

When creating a meeting on your calendar > Click on more options > Roles > Make the other person co-organizer (They must be under invited people first).