Concatenate multiple row data in single cell -only after a search character("1")

Hi Team,

Can you please help me a logic to transform the below mentioned data format:

Column A
1
AA
BB
CC
1
DD
EE
FF
GG
HH
1
DD
AA
UU
XX..

1 being a character to differentiate set of data.
XX being Usernames

I want the data to be in single cell for each set of data. Example
In B1 the data will be AA
BB
CC
In B2 the data will be DD EE FF GG(with alt+enter) ie new line but in single cell.

Please help its urgent, I have 9000+ usernames to collate.