Having more Co-op friends can be very enjoyable and beneficial on the farm. They can help you with your daily activities and tasks in the game. If you are the leader of the Co-op here is what you can do.
1. Tap the Settings button (gear icon) at the upper-right part of the "My Co-Op" tab to open the Settings Menu.

2. Once you have accessed the Co-op settings, you can either choose to set your group to private or public. "Off" means that your group is set to private. The leader will need to approve first the player requests before they can become a member of the Co-op. While "On" (set to Public) means that any player requests will automatically make them a member of the group.

3. Set the minimum level requirement: Players must have met this requirement first before they can enter your group (if set to "Public"). If "Private", the player requesting must have met the level requirement and will still need approval from the Co-op leader.

Note: Level requirements can be set to a specific level depending on the lowest level of a member that you have in the group. For instance, if the lowest level of a member of the Co-op is 28, then you can set the level requirement to 28 only as it's maxed requirement. Setting the level requirements to a higher level will also lessen the numbers of players that can join your group, as it will only accept requests from those that have met the specified level.

Another way to add more members to the Co-Op is through "invite", which can be located at the upper-left part of the "My Co-Op" tab.

The "Invite Friends" tab is limited to your Facebook friends only. You can either choose All Friends or friends that have an existing FV2 game on Facebook.

Note that if you tap a friend profile pic and you did not get a pop-up option to "visit Farm" or "view" / "invite to Co-op" option, this Facebook friend does not have an existing Farmville 2: Country Escape game yet.

You can either select all and send your invitation, Or you can either tap on each profile picture to find out which ones already have a Co-op group or which ones don't. If a friend already has one, you'll be shown an option to view their Co-op. If you don't have one yet, you'll see the "Invite to Co-Op" option. You can also send an invite when in the marketplace by simply tapping on the profile of a farmer.