Looking through the api the following jumped out at me as possible systems to look at ..
In Retail you should be able to use this for target/focus/mouseover .. theoretically
https://wowwiki.fandom.com/wiki/API_UnitChannelInfo
CancelSummon() - Rejects a summon request.
ConfirmSummon() - Accepts a summon request.
https://wowwiki.fandom.com/wiki/API_ConfirmSummon
GetSummonConfirmAreaName() - Returns the name of the area you're being summoned to.
GetSummonConfirmSummoner() - Returns the name of the player summoning you.
https://wowwiki.fandom.com/wiki/API_...onfirmSummoner
GetSummonConfirmTimeLeft() - Returns the amount of time left before the pending summon expires.
Events:
"CANCEL_SUMMON"
"CONFIRM_SUMMON"
"UNIT_SPELLCAST_CHANNEL_START"
"UNIT_SPELLCAST_CHANNEL_STOP"
"UNIT_SPELLCAST_CHANNEL_UPDATE"
"CURSOR_UPDATE"
Fired when the player right-clicks terrain, and on mouseover before UPDATE_MOUSEOVER_UNIT and on mouseout after UPDATE_MOUSEOVER_UNIT. This excludes doodads, player characters, and NPCs that lack interaction.
I doubt these will do anything remotely connected to what you want but it may lead you into a direction that may help you .. but ..
I have a sneaking feeling that the information regarding the assitant summoners and the summoning portal itself is not accessible in any way by addons. However, you the person being summoned can identify who is trying to summon them ( probably the warlock casting the spell ). The Channel Update even may have information relevant to how many people are needed .. IE there may be a numeric field that changes as people interact with the portal and if so you could use that to monitor how many .. but not necessarily who.
Most of these however, will probably only work in Retail due to the restriction in Classic of the UNIT_ events and unit related functions.
The warlock would probably be the best person to test these ideas out in case there is information only available to the caster of the spell.
As has been pointed out you may have to monitor the event system and see if anything of relevance is noted.