Sharing mission plans can be done in several ways and depends on the permissions you want to grant the other users.
You can share a mission plan as view only, or you can share a mission plan that can be edited by multiple users.
Share to a Contributor with Editing Permissions
Step 1 - Select Share from Mission Plan "..." Menu
Step 2 - Create a New Share and Select Contributor
Step 3 - Send Share Invitation Link to Other User
Change, delete, or create new shares.
Share with View Only Permissions
Select Viewer in Step 2 above.
If your mission plan is Public, by being in a public repository, you can also share the mission plan using the URL.
Share Public Mission with URL
If the mission plan is in a public repository, you can copy the mission plan URL to share with other users.