Liam Colgan, 29, organised the trip to Hamburg for his brother Eamonn's wedding next month, where he is due to be best man.
Another man in the group of around 18, Alan Pearson, said his disappearance was "completely out of character" and that "nobody realised at the time".
Mr Colgan was last seen at 1.30am on Saturday in the Hamborger Veermaster bar in the Reeperbahn area of the city.
Around six feet tall with ginger hair, he is understood to have left the venue a few minutes before the rest of the group.
He was wearing a brown leather jacket and a grey hoodie.
After he failed to turn up for activities planned for Saturday afternoon, police were called in.
His father and brother, and a few others, have remained in the city to help with the search.
Friends at home in Inverness have also been contacting hospitals.
The Foreign Office said it was "in contact with the German police" and is providing "advice and assistance" to Mr Colgan's family.
"It was towards the end of the night that Liam became separated from the group, but nobody realised at the time, given the nature of the event.
"It was hard to keep tabs on everyone," Mr Pearson told BBC Radio Scotland.
"Having retraced our steps and spoken to bar staff in the last bar he was seen in, we understand that he left a few minutes before the rest of the group.
"He's not done anything like this before. You might often hear in appeals like this that the behaviour is completely out of character, and that is absolutely the truth in this case."
A post in a specially set up Facebook page said: "Liam is the best man for his brother Eamonn and is responsible for organising the bachelor farewell in Hamburg.
"Eamonn wants to marry his fiancée Susan in three weeks in Scotland, so we're starting this call to find Liam."