Hello Appery,
From my experience it would be nice to have an option to create separate user accounts with "tester role" only.
This role would NOT BE ABLE to make any changes in the app. This option would be only for facilitating app testing phase (especially in case of small dev teams or single developers) since such users could easily install Appery tester from App Store or Google Play, use provided user credentials and perform app testing on his/hers device.
If I were you I would also consider not making this option dependent on the plan, and I also would allow to create freely at least 10 such accounts.