The phrase "activity has grown" is correct and usable in written English.
It can be used to indicate an increase in the level or amount of activity over a certain period of time.
Example: "Over the past year, the activity has grown significantly in our community, leading to more engagement and participation."
Alternatives: "activity has increased" or "activity has expanded."
As the activity has grown, so has the urge to institutionalize.
Manufacturing activity has grown briskly since last summer, accompanies by a steady rise in business confidence.
The likely perpetrator was the regional al-Qaeda affiliate, the Islamic State of Iraq and Sham (meaning Greater Syria), whose activity has grown explosively in recent months.
The activity has grown from small beginnings into a vast and complex industry responsible for the dissemination of all manner of cultural material; its impact upon civilization is impossible to calculate.
Eurozone factory activity has grown at a slower pace after manufacturers raised their prices for the first time in eight months.
The report says that construction activity has grown for 15 consecutive months and that last month hiring in the sector grew at its fastest rate since the survey began in April 1997.
He said such activity had grown progressively worse in recent weeks after the number of migrants in the area had increased from about 600 in January to its current estimate of more than 5,000.
When TSMC reported last month that its fab utilization rates, a key indicator of manufacturing activity, had grown from 67% in the first quarter to 85% in the second, it appeared to be evidence that the hoped-for industry recovery might be taking hold.
(In certain ways things aren't all that different today, though the cushioning etiquette around such activities has grown subtler).
The investigation of Senator Torricelli's personal and political activities has grown into a major crisis for him, legally and politically.
