Invite contacts to Microsoft Teams Free

From the menu: 

1. Tap your profile picture.
2. Select Invite to Teams .
3. Select the method you want to use to share the link.

From a chat:

If you sync your address book, you will be able to invite contacts directly from a chat. 

1. Select the Chat tab.
2. Select New chat  .
3. Enter the name of someone in your address book, phone number, or an email address.
4. Enter a contact name for the person you want to invite, then select Add.
5. Select Send Invite; the link will be sent to that person.

From the Chat tab: 

1. Select Invite to Teams at the bottom of the Chat tab.
2. Once selected, it will confirm Link copied to clipboard that you can share with your contacts.

From the Search window:

1. Enter the phone number or email address of the person you wish to invite.
2. Select the phone number or email address when it appears below the Search window.
3. Enter a contact name for the person you want to invite, then select Add.
4. Enter a message in the Type a new message field, then select Send . Your message and an invite link will be sent to that person.