Sharing an Item with item level permissions

Hi Bilal. thanks for the response.

We have a list of defined Members that need to submit items, in list format, for approval & tracking of activities they are working on. Most of the time, only the creator and approvers should have visibility to the items created. However, for select items, a creator may need to share the item with another member as both have ownership over the activity. The secondary member could vary however, is part of the Member Group that is allowed to contribute to the SharePoin List. 

Using examples to describe the issue: Jack submits an approval request for an activity by entering an item with the details. Only Jack and the approvers should have visibility to this activity.  Mary submits 3 requests for activities she is working on however, 1 of those 2 activities needs to be shared with Barry as they are collaborating on the activity, the other 2 should only be visibile to Mary. When Mary selects 'Items' --> 'Shared With' , it indicates it's 'Shared With' all members and approvers that have the abiloity to Read/Contribute/Edit etc. on the SharePoint.  Barry and Jack however, cannot see what Mary had submitted because the Item Level Permissions for the list are set to "Create items and edit items that were created by the user" (which is what we need most of the time). We don't want to change the Item level permission because then Mary, Barry and Jack will be able to see all items submitted.  So we need the flexibility to restrict/share on an item level basis. Based on what someone described what an Activity Directory allows, I would say this is what we need however, we don't have the skills or expertise to go this route if it involves any coding or developing etc. If it's easy enough on O365, I am open to giving it a shot but I'm definitely not a techy person but trying to help achieve what the business requires without IT resources.