I think the question is pretty self-explanatory. Would Prophet Muhammad (PBUH) be present (i guess spiritually) in a mehfil zikar? I have seen people decorate chairs or sofas in such ceremonies saying that the prophet will be among them.
Let's look at what you are asking, Is the messenger of God, Muhammad (SAW) present somewhere? You then elaborate, "Would Prophet Muhammad (PBUH) be present (i guess spiritually)...". I take that you find it obvious that his physical and bodily presence is not the question. Consider,
"Who perfected everything which He created and began the creation of man from clay." Quran 32:7
"Then He proportioned him and breathed into him from His [created] soul and made for you hearing and vision and hearts; little are you grateful." Quran 32:9
"And present to them the example of the life of this world, [its being] like rain which We send down from the sky, and the vegetation of the earth mingles with it and [then] it becomes dry remnants, scattered by the winds. And Allah is ever, over all things, Perfect in Ability." Quran 18:45
"Every soul will taste death .." Quran 3:185
"Every soul will taste death. Then to Us will you be returned" Quran 29:57
These verses show that a human/naas (as per Islam) is seen as a combination of flesh (body) and soul (spirit, or whatever you name it). Clearly, the body (as mentioned above) dies and is dissolved like organic matter. We understand the biology of body thoroughly, but before we answer if the messenger of Allah, Muhammad (SAW) is present in a gathering, "in spirit", ask the people (who make this claim) what they mean by presence in spirit. Quran itself says that humans can not understand the idea of spirit or soul,
"And they ask you, [O Muhammad], about the soul. Say, "The soul is of the affair of my Lord. And mankind have not been given of knowledge except a little." Quran 17:85
The "little" being the knowledge already present in Quran. Then how do the attendees of such gatherings make claims of anyone's (in) spiritual presence when they have not been given of knowledge except a little. As a general rule, no one should make any claims about something they know very little of. Messengers of Allah are humans and so was Muhammad (SAW). Abu Bakr (RA) said at the death of Muhammad (SAW),
“...'To proceed, if anyone amongst you used to worship Muhammad , then Muhammad is dead, but if (anyone of) you used to worship Allah, then Allah is Alive and shall never die ...” Sahih al-Bukhari Book 64, Hadith 472
Then Abu Bakr (RA) recited,
"Muhammad is not but a messenger. [Other] messengers have passed on before him. So if he was to die or be killed, would you turn back on your heels [to unbelief]? And he who turns back on his heels will never harm Allah at all; but Allah will reward the grateful." Quran, 3:144
So, it is absurd at best to say that someone, let alone Prophet Muhammad's (SAW) spirit is present somewhere and then leave a spot vacant for it. Spirits require chairs now? What if two such gatherings are held at two different places at the same time? Is the spirit present at the same time at different places? As you can see, this gets silly as soon as you start to contemplate on what does it even mean to claim the presence of someone's ruH/soul/spirit.
The object of a muslim's reverence and adoration is not the physical presence or the mystical spirit of a Prophet as clearly quoted above. We must revere and adore the message of Allah's messengers, and the truth in it.