I would say “infiltrate” implies that:

1. There is a relatively large group of people who
2. Coordinated with each other to
3. Join an existing political party which
4. They would not (or would not all) have previously had strong close engagements with in order to
5. Try to influence policy with respect to a specific area or areas of interest.

For 1, “relatively large” merely means enough to create a highly visible caucus within meetings. 1 or 2 probably wouldn’t do it unless the group being infiltrated was very small. In a large meeting, even 10% would be relatively large. In smaller groups, a higher percentage might be needed (1 person in a group of 10 is easier to dismiss as a crazy fringe than 100 people in a group of 1,000).

2 may be very difficult to prove. 4 even more so.

4 could include voting 1 that party, engaging informally with them in social media or IRL, volunteer work, etc. But mostly it’s an internal feeling that would be literally impossible to prove.

You can’t necessarily prove it absolutely, but even proving 2 alone provides pretty strong circumstantial evidence that the rest of it is quite likely.