It depends a bit on what you want to do.
On Geni you can work together with other users to expend your tree and connect it to the Geni World Tree. As a basic user (free) it is a bit more difficult to handle duplicate profiles but it is possible to work on Geni without having a paid subscription (it does works faster and easier if you do have a paid subscription)
On MyHeritage you are a bit more restricted in the number of people who can work together in building a tree and there is a limit in the number of profiles you can add if you are a non paying member.
For Geni you may want to look at https://www.geni.com/projects/Geni-Start-Here-Basics/17603 and https://www.geni.com/projects/A-to-Z-of-Help-Topics/38243
You can use SmartCopy (see https://www.geni.com/projects/SmartCopy/18783) to copy over data from other sites to Geni (do NOT use a GEDCOM import on Geni as that will create duplicates for all profiles already on Geni and it is a lot of work merging them and solving the conflicts)