Only Crew leaders can invite people to join the Crew. Directly after you have created your Crew, you will be prompted to 'Invite Members'.
Alternatively you can do this by going to the:
- Crew section > Your Crews > click relevant Crew icon > click 'Members' > select 'Invite Members'
- Your profile page > click relevant Crew icon > proceed as above
- Profile page of any user > click 3 dots in top right-hand corner > select 'Invite to Crew'
You must be following a user before you can invite them to your Crew, so you may receive a prompt asking you to follow them if you aren't already.